How old are the chicks? The younger the better.
I've done it a lot, almost always successful. Take the chicks down there after it is well dark and put them in the nest with her. You don't have to put them under her, just in her nest. They will find their way under her. Remove the other eggs before you put the chicks in there to make room.
If you want to put the chicks under her you can but be gentle. Sort of cup it in your hand and lift her with the back of your hand to make room. Be careful that you don't crush another chick already under her.
Are you sure those other eggs aren't going to hatch? My broody hens often hatch eggs two full days early, others say they have had hatches that late. There are different things that can cause that. Also, are you sure you are counting the days correctly? It takes 24 hours for the egg to get a full day's development after you set it so you say "1" when counting the day after you set them. An easy way to check is that the day of the week you set them is the day of the week the 21 days are up. If you set them on a Tuesday the 21 days are up on a Tuesday. But 21 days is just a general target, even with a broody hen.
Good luck. As long as the incubator chicks are only two or three days old it should work.
