I haven't hatched under a broody in several years, but surely could. Seems like every day I have another hen or two go broody this time of year. As of yesterday I think I have about 14 sitting around clucking.
I think too often the temptation is to put too many eggs under a broody. When you do that the eggs on the outside edge get cool and die, then when the hen rotates her eggs, the now dead egg gets worked toward the middle and a new eggs gets the cold shoulder. Over time many if not most of the eggs will eventually get cooled in this manner. You will produce more chicks by starting with less eggs. What that number is, is the question. For a large bodied hen I might go 8 to 10. She might be able to cover more, but it has been my experience that you also tend to get more broken eggs in the nest when the nest is over full.
I always went out with a flashlight after dark and candled a couple times, maybe at 10 days, and then again before hatch, and pulled any clear eggs or quitters. An ounce of prevention kind of thing, seen some nasty, nasty, messes when a bad egg busted under a broody.
As too adding chicks at hatch, I've done it, but I have also had hens kill some of the added chicks if they came out of an incubator. I seems that chicks hatched under a hen already know the lingo at hatch, from hearing the hen through the shell, or picking it up during hatch. These machine hatched chicks sometimes don't immediately respond to the hens calls, and depending on her temperament, I have seen hens peck and sometimes kill these chicks that don't respond properly.
