Absolutely. The first concern when thinking about introducing an adult chicken into your flock is where they came from. When you select a source, should you decide on an adult, you need to grill the flock owner on the flock health history. Ask what diseases they have encountered in their flock all the way back to the beginning. Some diseases are carried in otherwise healthy chickens who may never get sick from it, but can pass it on to other chickens. If they aren't sure, ask how many unexplained deaths have occurred in their flock over the years, and ask how they started their flock - chicks or adults?
It shouldn't be a problem, once you've done the health research and quarantine, to introduce one hen to your current hen. The problems arise when the new hen has to deal with multiple relationships and the subsequent challenges. One on one is quite simple, therefore it won't require much time for them to agree on who's top hen.
But as aart pointed out, you never can know when there's going to be a personality conflict. If that happens, you should know within a very short time after you introduce them to each other. Usually, the home chicken pecks the newcomer on the head, which lets her know she's going to need to fall into line behind her, and it's over with.
I wrote about a recent addition to my flock and how it went, and what I did to try to make it succeed. It's the third link below this post.
