If you want a bladed weapon with all the stats of a lightsabre, then just go narratively and call it a hilt attachment or something.🙂
Buuut it sound like a force relic.
a normal lightsabre is at a great disadvantage against cortosis, but your sword will only be slightly worse off against cortosis. (It is still an ancient sword + brawn damage) (and the blade probably needs to be cortosis as well to handle the plasma field around it)
depending on the campaign. Will it look enough like a lightsabre in action? And will it attract the empire or inquisitors?
Or go the Je'daii / nightsister sword enhancing way, and invest xp into learning the awesome samurai to actually enhance a blade with the force and his dedication.
Sounds more jedi samurai to me 🙂