Can you add a duck to a pen where one duck has always been by herself?

generally, yes. Many ducks like to have a friend and will get used to having someone else in the pen with them. at first, she may get a little territorial, but that is ok! she is just developing a pecking order and letting the new duck know who's boss. If the new duck is quite small and is getting picked on too much, I would wait until it is about 3/4 the size of the adult duck before you introduce it into the pen. That way it can stand up for itself if it has to.