Never tried it, althogh I doubt it.
In theory, first u'd have to edit the Race.ids file and Race.LUA (Race.2da) and then use a script to change to that race because I can't think of any way to add it to the options screen like u can add kits.
I have changed races to existing races with a script but never to custom, I have been meaning to try it, but as I said, I doubt it is possible.
