Well, I don't think you can add another race into the race menu when you are in lobby, but when the map starts, you may show a dialog button, asking if the player wants to change his race into one of the custom ones.
E.g.
In lobby it matters not which race you choose, but when map starts, the dialog window could have something like:
"Do you want to play another race?" with dialogs buttons like
- no
- demon
- naga
etc.
Then if you choose any button for the new race, you replace main building and workers with the main building and workers of the new race.
This will be bad only if you play as undead, as the ground will be permamently damaged by blight (or dirt when blight is removed)