You want to be able to change appearances of the in-game characters? That is doable by editing the .utc of the character in question. Use KotOR Tool, and use the Creature Editor. Quite simple really
EDIT:
You will need to save the new creature and put it into your override. At the same time, you don't want to destroy the original characters unique template. So, make a copy of, say the p_zaalbar.utc, and rename it old_p_zaalbar.utc. Leave this unaltered version in your override if you like, or some other safe place. Then, edit the p_zaalbar.utc with the appearance you like, Alien_Gamorrean01, and save this edited version to your override. Now, when the game renders Zaalbar, it will use this edited version in your override and make it so Big Z is a big pig face.