Yes, it is rare that quail go broody. Conditions need to be perfect in the environment and in their own heads. It is a delicate combination. Keep in mind, some birds can set for 10 or so days and then abandon ship! So make sure to give her as much space as possible, lots of quiet and alone time and don't mess with her or her surroundings as much as possible. Keep the predators, dogs, cats and kids away as well.
As for adding previously hatched chicks, if they were day old you could slip them in during the night and she probably wouldn't know the difference, but week old chicks, not likely, especially if she does have day old babies. She will need to attend to her's first and would cause too much commotion.