People slip store bought chicks under mamas all the time, usually at night. They adopt other chicks just fine, even other species -- I was just looking at pics of a mama with baby chicks and baby keets , have also seen this with ducks. They will also set duck or keet or whatever eggs and hatch the babies. I have given store bought chicks to a mama with chicks of her own in the middle of the day and have them accepted well.
I just leave the mama with the flock during setting and to raise them. I have a chicken wire pen in the coop built for broodies and new babies, but once the broody is setting on real eggs, it is very difficult to move her and have her stay on the nest, so I have to isolate her in there for a day or two before allowing her to set eggs. Some people have been able to move a setting broody with eggs successfully, at night, but it has not worked for me. One batch I let hatch in the broody pen. Trouble was, the new chicks could get through the wire, and did, leaving mama in the broody pen and the babies out with the rest. So I left it like this for a couple of days; the chicks would go back and forth to mama all day long. Then I just gave up and let mama out. Haven't fixed this problem and tried keeping them in the broody pen yet.
I did let a few hatch this year in the heat of the summer in the coop, but mama turned out to be a chick killer. I used to have a wonderful mama who did a great job of raising all her babies in with the flock. Wish I still had her.
It's such an individual thing; you just never know how it's going to turn out. In the old days, of course, they were always raised with the flock. And I'm sure the ones that survived were the strong ones, ones you would want to breed again.
I put 4x4's and bricks and stuff down so the babies could get to the nest or whatever, but after a couple of days of running around, they were able to jump/fly about as high as they wanted. At two or three weeks they had no trouble flying up to the 30" high roost. They didn't roost, they slept under mama in the nest, but they played on the roost. They only used my steps a few times before they were going up and down on their own. People have raised chicks who were hatched in hay lofts. They half fly down on about day 3 and everyone finds a different bed on the barn floor.
