Introducing Social Hamsters to Each Other
If you do want to introduce a new hamster to a group, it is a good idea to remove all the hamsters from the cage and temporarily hold them in individual cages while you prepare the old cage.
Wash and disinfect the old cage and all its furnishings, and then rebuild the cage with items arranged differently. Make sure you have more than one hiding space per hamster (such as paper towel tubes, plastic igloos, or boxes) and multiple water bottles and feed stations (ideally one water bottle and feed station per hamster). Once this has been done, place the new hamster into the cage first, give it about 15 minutes to explore, and then add the other hamsters. Do this at a time where you can closely monitor the cage for fighting for a few hours and periodically inspect each hamster over the next several days. If you see scabs, weight loss, hair loss, or diarrhea ("wet tail"), please separate them and have them seen by a veterinarian. Some hamsters just never get along so think twice before you try to add another hamster to a well-established group. There's a chance you might end up with several cages of individual hamsters instead of a happy group!