That wouldn't really accomplish anything.
that would sort of be what you're looking for. make the first butler a zombie via making him selectable. in the next household cancel the zombie and it will spit out another human butler, change him to what ever you want. it's a little time consuming, but anything worth while is. on the other hand i think doing it this way gives you an extended "friendly" pie menu with all the options of a visiting sim.
i never thought to make the service sims anything else, but now i'm going to have a plantsim gardner.