Do you mean you added an egg a few days after they started sitting on them? That means staggered hatch and can make some difficulties. I would suggest separating the broody hen so she doesn’t get kicked off the nest or so eggs don’t get busted by others.
Exciting! I would be a bit concerned with it only being one egg though, you may want to have a backup ready with other young chicks for buddies since chicks were meant to have siblings. Good luck keep us updated.