Q: A chicken will hatch eggs that aren't hers??
A: YES! She will sit on balls, eggs, and any other roundish shape she can find! You can even give her duck eggs to hatch!
Q: Will she care for the chicks all by herself?
A: If she is a breed that is known for being a good mother then she will hatch and care for the chicks all on her own. Just make sure the chicks have food and water available. If she is a breed that is not known for good mothering, then you need to be preapared to take the chicks when they hatch and raise them your self.
Q: Can she join the rest of the flock in the main coop right away?
A: No. She needs some time with her chicks on her own. She would protect her chicks from the other hens but you can never be sure.