Rebrascora is correct. 21 days or not, if the hen is not trying to hatch the eggs and you stick a live chick in there she will kill it. A broody hen sits on the eggs all the time, barely leaves to eat and drink. She doesn't lay any more eggs and is trying to hatch the ones that she has under her. If the hen is not sitting on the eggs 24/7 then she is not broody. The other issue is the hatching event itself. The chicks don't all of a sudden show up. For a couple of days before hatching, the chicks are peeping inside the egg. The mother hen hears this and talks back to the chicks .. and they answer. This happens to imprint the mother hen on the sound of the chicks before they even hatch and vice versa. She also watches them hatch and removes bits of egg shell from the nest. All this interacting strengthens the bond. If you honestly have a broody hen, wait you 21 days and see if you can manage to get a chick under her without making her leave the nest. Best to do this at night. If the chick stays put, it might be accepted. But I would be vigilant and watch. You might have to remove it if there is any aggression. You can attempt it but expect to be raising the chick yourself for several months until it is old enough to join the flock on it's own.
