Congrats on your broody!
As BlindLenonChicken said, having a backup plan is important. Nothing is guaranteed when working with animals, and especially first time broodies.
I have had both positive and negative experiences with grafting/chick adoption. I much prefer giving the broodies the eggs at lockdown, but since this is not possible in your case, the earliest you can get them is when you can try.
Again, as Blindlemonchicken mentioned, within twenty four hours of hatching is when you’ll be able to do it.
There are two reasons as to why that is. The main one, is that the new mother imprints on her chicks within that first 24 hours, so any new additions after the imprinting process is complete might not be welcome. Again, that is not a guarantee, but the vast majority of hens will react violently to outsider chicks.
The second reason, is that around the same time the imprinting process is complete, the hen will take her brood off the nest. She leaves any chicks that haven’t hatched behind, and once again, most hens will not accept new chicks after this point.
24 hours is more of a rough estimate, though. Even having the same girl brood twice, you’ll find how differently things are done. Some hens will wait closer to 48 hours, while others won’t even get to 20.
Hens with smaller batches tend to imprint quicker.
Inexperienced broodies also take longer to leave the nest. This should work in your favour.
Keep in mind that chick adoption requires both the mum and the chicks to be receptive. The latter usually are, but older chicks such as the ones you will receive in your shipment will be quite eager to explore, leaving the nest and causing problems with the imprinting process. They also run the risk of getting attacked by the other adults in the coop. I would strongly advise blocking off the new mum and the chicks from the coop, and keeping them in the nest box.
There is certainly room for things to go right. As long as you have a brooder ready for the worst case scenario, then things should be ok
