@Nheko has one of the sources I would recommend. The L5R 2nd Edition supplement Way of the Daimyo also had an appendix about creating Minor Clans.
Honestly, if they ever make an L5R Foundry for the RPG, I would totally write up a supplement to make a "20 Questions to Create a Minor Clan" guided play module for campaigns who want to have founding a new Minor Clan as a central theme.

Yes, Secrets of the Empire, however was part of 4 Edition. Still you can use part of the fluff and ideas to do yours.

RPGs are at their peak in the US. I will guess that this is the decision of Asmode, a French owned company, who doesn’t know how to market well. Look at Games Workshop sales in the US ( very high ) and Wizards of the Coast with D&D sales and tell me the market is not there. The L5R RPG is a wonderful variation on the tired and traditional western fantasy RPG tropes. It will be a tragedy to see the line discontinued or diminished. All my opinion of course.

Using two opportunity symbols to inflict a critical strike on the same blow (roll) that put your opponent over their fatigue value might be that “finishing blow” and that’s in the RAW according to most of the responders to this thread ? I get your point however.
As the GM, I would have such incapacitated opponents concede defeat or run away or yield, being within the bounds of the Social code of Rokugan ( Shadowlands creatures and hardened criminals excepted ).

It wouldn't really fix the end of the fights. It still would be "stab them while they are unable to do anything" type of gameplay.

I "kind of" understand the stabbing of Unconscious characters, ok they can't do much they are KOed, you want to finish them..
But the free stabbing of Incapacitated characters who can run around and talk and keep doing whatever mundane activity they are doing? Is just ugly.

One should be able to do a finishing blow when the other character can still retaliate.

IF you are able to make that chance of retaliating lower (increased TN, compromised, bleeding etc) then it is even COOLER.
But making it "impossible", as cheaply binary as that... is bad design. Outside of the character being KOed, one should never be unable to do anything with a check but otherwise be fully functional and unwounded. You need to leave that little glimmer of hope in there, that 1%, whatever, so that a character can make a last ditch effort, no matter how difficult the odds are.

 

In fact, against low-Deadliness weapons, it's often a very good idea not to Defend, because then you stay in the fight.  If your Fatigue is close to your Endurance, and your enemy hits you with a tetsubo or otsuchi, Defending against that hit is going to leave you Incapacitated for several rounds.  While you're taking that Calming Breath to recover 1 Fatigue each round, he's going to draw his katana and finish you off.
You're better off spending a Void Point to not Defend, and take that Severity 3 Crit.  Assuming you have any fitness at all, and are using one of your better stances, you should be able to turn that crit into some dented armor or ripped clothes.  Even if you botch that roll, you have a Lightly Wounded Ring and you're not even Bleeding.  Change stances and mess that fool up.

Incapacitated for multiple rounds, unwounded, best part of the game right here. very cinematic and fun.... /s

Yeah.  You take damage, you either suffer a Crit, or Defend and suffer Fatigue.  Attack over.
That Fatigue can put you over your threshold, so you can't Defend against more damage, but nowhere does it say you suffer a Crit.  You specifically Defended so that you do not suffer a Crit.

"A character can still defend if defending would cause their fatigue to exceed their endurance." (pretty self explanatory, you are still Defending)
However, having fatigue in excess of their endurance does render a character incapacitated (see page 272), leaving them unable to defend against future damage!" (they mention "future" damage. again, making it pretty clear)

@Avatar111 is correct.
Any time your fatigue exceeds your endurance, you are incapacitated. as an aside, Endurance 9 is technically impossible for a PC, as it's (Earth + Fire) x 2 - so must always be an even number greater than 4 You do not receive a critical strike for being pushed past your endurance. You are, once Incapacitated, unable to defend, so the next time you suffer damage, you will automatically receive  a critical strike and hence, as per the Incapacitated rules, become Unconscious.  
 
There is a possible issue if a PC is rendered incapacitated by a strike action with kept results , with the results spent to inflict a critical strike.
The Strike action's opportunity is ": If you succeed, you inflict a critical strike on your target with severity equal to your weapon’s deadliness."
That implies that whilst the opportunity is spent normally, the critical strike does not take effect until you resolve the  symbols, which is when success is determined and which applies the 'success' effect (the damage which will render the target Incapacitated).
It's not entirely clear, therefore, if the fatigue or the critical strike would be resolved first in this case (or who decides if it is a choice). This is important because applying the critical strike to an Incapacitated opponent is obviously far more dangerous, since it would automatically render them Unconscious.

the Bushi defend and go to 11 fatigue, become incapacitated, but take NO critical strike

There is only the kiho: breaking blow.
But nothing stops you from adding a way to break weapons. (This game is houserule friendly) Maybe make it opportunities = to target's vigilance on a succesful attack and deal no damage from the attack? Each 2 extra opp add another damaged quality (destroying the weapon).
A disarm would be in a similar category?
Anyway, come up with something that is generic and not as easy to perform as the technique.

Yes. Since it starts with Damaged, it will become Destroyed the first time it's used to parry, or strike an armoured opponent, or what have you. Also note that the Experienced Bandit will have a TN+1 penalty to attacking with it.
(fortunately, he has a Kusari-Gama which is a far better weapon for him, since he wants to stand behind other bandits and yell Get Them! a lot)

There are a few techniques - like Breaking Blow - which can forcibly apply damaged to a weapon, but by and large, they don't.
Anything which can give a weapon the damaged or destroyed quality will specifically say so; there's no 'default' effect.
Note that a few NPCs start with weapons and/or armour with the damaged quality (generally bandits, goblins and so forth), and the GM is of course at liberty to narratively declare the quality now applies (after a couple of straight days of hard fighting, for example, it's not unreasonable to say 'you're going to have to find some time to maintain your weapons and armour, which narratively speaking means 'it's now damaged')

At whatever point you port the character from beginner rules to the more advanced system, just "recreate" the character as if from scratch taking note of experience points spent during the 20 questions. For preferred access skills etc through schools, just buy them at the earliest moment possible. For the whole process, I'd recommend using the Paper Blossoms character creation software. Note that different PCs might start out with different XP totals, just award out the difference so everyone begins at the same amount. I'd recommend adding extra to account for downtime scenes since the end of the beginner box in your timeline.

In that case, you may have to make them aware of you need. As well as communicate the intensity of your need for such a ruling.

Using the official contact form has worked quite well in the past. It's not super fast, but does get answers in general. 

Waiting for an official FFG ruling on this one. Thank you for the replies!

I allowed my players to create whatever they wanted and we ended up with 4 different clans represented.  The way I got them all together was to have a mysterious samurai arrive at each of their genpuku celebrations and speak with the diamyo.  The PCs each in turn were told that they needed to fulfill an obligation on behalf of the diamyo and were given letter with instructions and sent on their way.  The letter included the names of each of the PC samurai and instructions to complete a task.  This got the group together, and gave them a mutual starting giri.  As an aside, the mysterious samurai was the jade champion (in my game the position was filled) and they group is well on their way to being recruited as jade magistrates. 

They're in the core book.
Hiruma scout is bushi/shinobi whilst Kakita duellist is bushi/artisan for example.
So far, the tags have no specific rules associated with them, but some techniques in sourcebooks like shadowlands have them as prerequisites. 

I don't think so, as they're not very important so far.
There's also Artisan.

Also shinobi

All of the previous answers are great but I wanted to add one thing. As a new player you're actually hitting a major theme right on the head; namely that it's less common for samurai in the broader sense to engage others outside their clan. For PCs participating in an interesting story, multiclan parties are a common exception, but remember that "independent adventuring parties" that you'd find in other rpgs don't really exist- unless we're talking about a band of ronin. PCs are tied to lords and it's less common for lords to command samurai across clans, and less likely that lords of diverse clans would encourage their noble servants to collaborate together without a purpose.

Another reason is a a shared mission.  Like if Clan A (or allied clans A and B) send two party members out to investigate and solve a big problem, and then get in trouble, Clan C (or allied clans C and D), who have been sent on the same mission/to solve the same problem, can find the others in trouble, rescue them, learn they have a mutual interest, and decide to team up together.  Pairing two pairs is easier than pairing 4 individuals.

Hostages and relatives, too.
Yes, Isawa Kaede is technically Akodo Kaede since she married the lion clan champion, but if she needed a party of samurai to go poke something, it'd be quite feasible to find the odd relative from her old family in the group too.
It's far from unheard of to have a samurai from another clan who lives in another clan's city - as a representative, ambassador, relative of a clan member (current or deceased) or an artisan of some kind who set up shop there for whatever reason (a Kakita smith who petitioned the local not-crane-but-not-hostile-to-the-crane-either lord to establish a forge there because of the quality of the local iron ore, for the sake of argument).
 
Obviously the bigger the 'problem', the more clans naturally get drawn into it. Bandits on the border of both clans are arguably dealt with by a force from both clans so there's no room for misunderstanding, gloating and hard feelings afterwards. A threat to the entire empire like a maho-sensei.....well, that's the entire Empire's problem to solve.