Well, members here always say you have to quarantine a new bird for a while so that you know they are disease free. So I need to point that out. However, alot of people don't have a separate place to put new birds. For me personally, if I knew and trusted the person I was buying the new hen from, I would just go ahead and put them together right away, especially if you only have just one established hen. I would try and get one the same age and size, because they will probably quarrel and peck at eachother a little bit at first. If you put a small hen with a big one, then the bigger hen can do some serious damage.
Try putting in a very tasty treat for your established hen when you introduce the new one. Maybe that will distract her and she won't be so upset that a new hen is in her territory.