I would strongly advise that you do not put any new hens with your originals immediately. Before you buy any new birds either purchase or make another house and run that you can put any new birds in as a temporary quarantine section. Ensure all your birds can see each other but not have any contact. Keep your quarantined birds in their new run for roughly 3 weeks just ensure they are not the carriers of any infectious problems. Try to feed your new and old birds at the same time and any scattered corn in the afternoon is down at the same time. Bring the new birds closer and closer over the three week period so eventually you are feeding them all almost in the same place but with wire between each group. This way you will ensure that when you put your new birds into the main hen house there will be the minimum of distress and stress caused to both groups. Try to imagine your next door neighbour shoving his smelly and irritating cousin through our front door and saying he is coming to live with you...how would you feel? At a guess not good!
It may appear an added expense but this quarantine house and run will serve you well later either as a future quarantine area, a hospital wing (excuse the pun) or a broodie coup.
If you put new hens in with your present hens immediately you are running a very real risk of passing on infectious problems, horrible, horrible, fighting and having all round stressed birds.
Best of luck and remember slowly and gently is the best option.
OL