first, it is posible to add new models without overwriting the existing. you simply need to download these models and put the pk3 files in the base directory. the models will show up like for ex. Boba Fett already does in JA multiplayer. by now i haven seen any purely JA models jet, but the models for JO should work too as i heard.
second, to get a mod or something like this for more heads, torsos and legs in the costum model creation is very unlikely. these body parts are not in single files, they are all together in one model file. if you want to add a new head to the human male, then you need to have the original model with all parts in xsi format, add the head and recompile it. this new file would need to replace the original one. its to much work and it would be needed to do for every new body part from different source.