First, be aware that even with quarantine, you may be importing serious disease into your flock, such as lymphotic leucosis which is similar to Marek's and has no vaccination or cure. Chickens can carry this virus and be asymptomatic, thus quarantine won't ferret it out.
The best thing is to question the source of these new hens and find out if there has been any history of unexplained deaths. Most chickens are resistant to many of these diseases, but can still pass them on to a new flock. You may want to turn down these chickens if you find out there have been mysterious deaths where they came from.
That said, I introduced a rescued hen to my flock last year, and I wrote about it for BYC how it went. You might be interested in reading how I went about integrating her. Photos included! It's linked below this post.