So the eggs should hatch in around 10 days, and she's been broody for a few days, so that's enough time. You could put the eggs under her now and let her hatch them out if you think she'll be a reliable broody. If she's a first time broody nine eggs might be 1-3 more than ideal, so you could put 3-6 under her and the rest in the incubator. You could replace her fake eggs with real eggs when she gets off the nest to eat, drink, and poop, if you ever see her do that. Or you can go into her pen at night and pull out all the fake eggs first, then put in the real eggs one by one. If you put them down in front of her breast she will likely pull the egg under her. I usually just put the eggs under her wing and she rearranges however she likes. If she's really tame and used to you handling her you could probably do this anytime day or night, but it very much depends on the temperament of the broody. Some birds will stop brooding if you mess with them.
If you do switch out eggs, it would be a good idea to have identifying marks on numerous areas of each egg. That is especially important if there might be other hens that could crowd in with your broody and lay additional eggs in the nest. If you draw a circle around the equator of the egg so you can see it when the egg is in any position, then you'll know if there's any new eggs being added. If there are new eggs, removed them asap. Too many eggs under a hen can decrease your hatch rate, sometimes significantly, and new eggs can create staggered hatches, which can end with either untended chicks, or more likely abandonment of near-term eggs prior to hatch. So if other hens can lay eggs in broody's nest, check the eggs daily or every other day and remove any unmarked eggs.
If you graft chicks from the incubator onto the broody, it is important to stick around for a little while afterwards to be sure that she accepts the chick(s), and that the chick stays in the nest and doesn't wander off. The younger the chick the better, but 3 day old chicks are probably the oldest that will graft successfully on a regular basis. There are exceptions, of course, but you can't count on those. It's not just a matter of will the hen accept the baby, but will the chick accept the hen. Chicks imprint on a caretaker during the first three days of life, whether it's a hen, a person, a stuffed animal, or a heat lamp. A 5-10 day old chick that knows to get under the heat lamp for warmth will look at the giant bird and think it's T. Rex, possibly run away, get chilled and die. And even if it stays under the warm feathers, it may not listen to the hen as it grows and the hen tries to teach it life skills. If it wanders off when the hen tells it to come back, it may get eaten by a cat or ??? So grafting a chick to a hen is a two way street which really should be started by day 3, if possible, although there have been successful grafts that have started later.
How to graft successfully does somewhat depend on the hen. Theoretically, it should be done at night after the chicks are dry. Use the minimal light possible where you can see, and don't shine the light at the hen's face. Also, don't drop the flashlight. A keychain flashlight that is attached to a lanyard around your neck works great, as does a headlamp (as long as you don't shine it directly at the broody, which I have difficulty avoiding with the headlamp). The dry chicks are placed under her fluff from behind, or under her wing if she tolerates handling, but not up front where she might peck an unfamiliar chick before it gets nuzzled it and she bonds with it. If there's enough light for her to see well, ideally cup your hand over the chick so she can't see it during the transfer.
That's what I've been told for the average broody, where there's the potential that she may not want chicks that aren't hers, so you have to sneak them under her to be sure that they will be safe. I started out that way with my hen. She had 5 hatching/hatched under her, and I had 12 eggs in the incubator timed to coordinate with her hatch. The incubator chicks were soooo active that I had to take them out of the incubator ASAP, as they were flopping around, rolling the other eggs around, and seemed distressed to not have anything to snuggle against as they dried out. So the moment they were half way dry I pulled them out, weighed them, leg banded them for pedigree, wrapped them in a warm T-shirt out of the dryer and took them out to the barn to put under the hen. I started out trying to sneak the chick in, but as the day progressed into night both she and I got tired and the pretense totally fell apart. By the end of the night I just walked into her pen, showed her the chick all wrapped up warm, picked up her wing and tucked the chick somewhere under there with all the others. By the end of the night her little "wingpits" were bloated with squirming peeping chicks, a total of 13. Way too many for a first time broody, but she's set up in a way where there are no dangers to the chicks, and it's summer, and she's amazing. All 13 are growing like weeds, and are 10 days old today. But that's a broody that is extremely tame, totally trusts me, and didn't look at these chicks as interlopers from another nest. Not all broodies will have that type of temperament, so sneaking a chick in during low light is definitely the safest way to go until you know how your hen will react to new chicks.
