Problem with RTW unit modding is you need 3dsMax which costs a small fortune
You can open the models using a script in GMax but can't export them in a format suitable for the game...

Have a look through info in the Scriptorium there's stuff related to models here:
https://forums.totalwar.org/vb/local_links.php?catid=82

and skins here:
https://forums.totalwar.org/vb/local_links.php?catid=75