The bonuses are hardcoded. You can make it so that any proficiency can have up to 5 stars, but the game doesn't know what to do with more than 2 stars (or 3 for dual-wielding) in the weapon style proficiencies.
AFAIK that should be possible. Are you trying to change an existing class, make a new kit, or what? You might check some of the tutorials out there, like this one.
Is it possible to devote more specialisation slots to the weapon styles? I changed the weapprof file so that i could give three slots to the one handed weapon style and the game did not crush, but giving three slots to the weapon style had the same effect as none slots. How can i change this, is it possible?