Note: for brevity, I will refer to the Linear d6 system as L6.
There are not really any established guidelines for translating from the d20 system to the L6 system. Since you asked, I did a little comparison on my own to see what the game designers may have down when recreating the charaters in the new system, and it seems that they have done just that, completely recreated the characters in the new system, often keeping only the background.
Very general guidelines would be to treat level or hit dice in the d20 system as the grade in the L6 system, but this is not universally adhered to. Cinderella, for example, goes from being a level 4 character in d20 to a grade 6 character in L6. The more significant and/or powerful the character is, the higher their grade will be. I would suggest graduating Red Riding Hood from a 3 hit dice character (d20) to a grade 6 character (L6), as she seems both powerful and strongly iconic (one of the more recognizable characters). On the other hand, I would translate The Frog Prince directly across from a 4 hit dice character (d20) to a grade 4 character (L6), as he does not seem as powerful as Red.
Speed is another area where the games diverge a little. In d20, a medium creature will have a speed of 30 feet. As this is an average sped in d20, it should translate to an average speed in L6, and in many cases it does; but not in every case. Cinderella, Jack, and The Ugly Duckling all have a speed of 30 feet in the d20 system. In the L6 system, Cinderella is Average speed, Jack is Fast, and The Ugly Duckling is Slow. There are lots of other examples, but I think the point is made that in L6 speed is not as much a fixed characteristic as it is in d20. Just use your best judgement, is the character faster or slower than average. For example, in d20, Red has a lot of feats which suggest quickness (Improved Initiative, Lightning reflexes, Mobility), so I would be inclined to give her a Fast speed.
Next your character needs to be given traits. Core traits may serve a similar significance to Abilities, but there are not really direct relations between each of the core traits, and each of the abilities. You will find a little more in common between Playground/Study Traits and Skills, but there is not a formula for translating a skill bonus to a trait grade, as the designers don't appear to have used one. Just look at what skills and abilities look to be above or below average, and grade them accordingly. For example, Red has a Charisma of 21 (the same as Cinderella). This appears to have translated into a Cool of 12th grade, and Cool as the character's iconic trait. The high Cool should also cover her high Trick skill. Her high Notice skill would equate to a high Seek trait.
Don't try to base any of the trait grades off of base attack or saves, as in the d20 system these are all a direct result of level, and not a real indication of skill; use your best judgement instead. You can use feats to give you an idea of where a character might has higher traits, but do not try and translate them into talents, as the NPCs don't generally have any talents. Instead they have special stuff. This is just like in the d20 system. Soft Spots and Vulnerabilities are basically the same thing, as are many of the abilities; however, this is a perfect opportunity to fine tune some of the abilities to better reflect the character instead of the system. For example, The Frog Prince is given a Telekinesis special ability to simulate his tongue. The L6 system does not rely on lists of pre-made spell-like abilites to define what its characters can do, so change this ability so that it becomes exactly what it was intended to be, a tongue. Something like this is much more appropriate than telekinesis: Tongue: The Frog Prince can use his long, flexible tongue to make wrestling attempts on a target up to a cricket's hop away.
Anyways, there are my suggestions. I did not simply post translated charcters because the game is still new to me, and I do not claim to have a full understanding of what impact everything is going to have on actual gameplay. Perhaps, after more reading, I might; but for now, my suggestions are simply based off of my observations from doing a comparitive analysis of the NPCs included in both games. Ultimately, the grades you assign are going to depend on how major, or how minor, of a threat you want that character to be in your story. Tailor it to your needs. I hope this has been helpful.