I am afraid that adding completely new feat to the game, or even rewrite or copy them it's not actually possible

This is so because the code that control the various feat is literally ingrained in the engine, without a source code they cannot be changed

Now, most feat have adjustable parameters from feat.2da table, and in NWN2 it's also possible to add new class of melee weapons, not ranged one unfortunately, but on the first NWN you can't even make those

The only way i know feat can be added to the game is by scripting, adding invisible supernatural permanent effects on a character, it this way it's possible to alter statistic, regeneration, skills, resistance and so on. Custom modules use this methods to make new custom feats, as it's possible to bind a script ID to a feat

Alternatively you can modify a feat bonus to stack with another, and then change the relative class feat table and add all those feat in, if they are class specific the stack bonus won't be gained by any other class anyway, as they can't learn both or more feats

In short you can make new feats, but anything rooted inside the game engine cannot be modified, while adding new scripts that affect the caster is entirely feasible. But if you want to add new feats that increase sneak attack, give more attack per round or even give more critical threat than it's not possible

The only thing that bind this hardcoded feat is their ID, and the CONSTANT column, but this second value doesn't seems to do anything on it's own