I seem to remember we've discussed this here before, but I can't find the topic for you; sorry.
I think that you would just make the tips out of epoxy or polymer clay and attach them with some glue(elmer's should hold and be easy to remove later). Of course, you would cover your doll's ear with plastic wrap while molding the ear tips so that they don't stick and become impossible to remove. With this method the join won't be as smooth as the permanent method, but you can cover it with hair or some jewelry.