I introduced three (two month old?) chicks to the flock last spring... I put a big dog crate in the coop, and kept the chicks locked in there for a week. Then, I would let them out a little every day under strict supervision. I eventually wired the door open with just enough space for the chicks to get in and out, but not wide enough to let the adults through, so the chicks had a place to escape to. I forgot how long it took until they were sleeping up on the roosts, but it wasn't more than a couple months.
I should say that I have mild-mannered Buff Orpingtons.