She might take it. My girl Thistle was on infertile eggs and I swapped them out for chicks at the three week mark. But, if you give it to her before the eggs hatch, you run the risk of her abandoning the eggs for the chick. At any rate, if you decide to do it, I would say wait until the eggs start to hatch or are due the next day, and then slip it under her during the night. Just don't wait until it's too old or it will not imprint on her. To summarize, yes, you can give a broody hen newborn chicks, but she might not accept them and kill them, and she might abandon the eggs she's on for it.