I'll try and sum it up then I'll give you a link that helped me:
You have to export:
-Model/texture bress You Want
-Animations You Want
and replace all files in an alloy with the files you exported.
Notes (as far as I know/taught):
The Model you want to use has to have more bones then the one your replacing.
Your Best bet would be to replace an alloy and use codes to prevent crashing and to enable there playability.
Where I Started My Trail:
Read the description.