1. You either have to use one of the Mods that have already been made, or you have to create your own Mod.

2. If you want to spin your own and have never made one before, then I highly recommend you go to the following:

http://pub24.ezboard.com/fshoguntotalwarfrm5

and read the thread entitled "Modding MTW". Starting on page 2, is step-by-step instructions about how to create a new playable faction.

There are also lots of Mods as well as Modding instructions and hints on other boards at this site. I'm fairly sure an existing Mod is available, I just don't know the names of them.