you have to assemble them inside the 3d editor (and pose the character), then assign every texture to every chargen piece using nifskope
but i assure you it is possible and not too hard to do (i actually got this working pre-ck), just annoying
http://static.skyrim.nexusmods.com/downloads/images/5696-1-1328385683.jpg
we dont have access to Bethesda's facial rig, so i dont think you can change the facial expression using bones, you would have to shape it manually by moving vertices