Based on my recient experience, using a wire cage to put the new hen(s) in inside of the coop will allow the older chicks to get use to the new comer. I reciently introduced two older pullets into my flock of young pullets and two young roosters. I thought all was well until I found bits of feather here and there and bald spots on the backs of two of my prior chickens. The older two were attacking the younger ones. They seem to be all getting along now, but I had to seperate them for a bit then reintroduce them slowly. I still have 10 chicks to introduce eventually so I am going to try the slow approach with them.