Try asking the question on the site I linked below. They're usually pretty active there and answer questions quickly.

I'm not sure if chickens are the same, but when I raised exotic birds, I could only put chicks with a hen when her eggs were hatched or due to hatch. Timing was critical since some hens, in a brooding state, wouldn't deal with chicks that needing feeding.

I would think your hen, while sitting on eggs, can't/won't leave the nest in order to teach the chicks how to feed. Or, she may abandon the eggs in order to take care of the chicks. But that's a guess............. : )

Here is a link that might be useful: chickens

It's not a good idea at all. The chicks are too old. They won't listen to her, and she'll most likely kill them. As Annz said, timing is critical. I have 3 setting now. One good setter and two young ones. Young ones are not good setter and your's seems awful young. I set all three hens on the same day. Will divide all the hatched chicks, AT NIGHT, between the three so all have something to show for all the lonely days and nights. I have hens actually kill chicks that are barely pipped. Still wet. They are pretty particular.
good luck and I hope she has a fine family. there's nothing more fun then watching a good mama hen raise her peeps.

Your best bet is to keep your chicks under a heat lamp. Once they feather out you can put them out with the other chickens (hens) . Take in mind any chicken that you are the caregiver for will inprint on you. This Spring I hatched out 18 , lost one at week old . my little ladies follow me around the yard. and Yes they come up to my door and think they need to come inside. wish you lots of luck.