Can I put day old chicks under her and will she accept them(and not kill them)?
You are dealing with living animals so anything is liable to happen. People successfully give day old chicks to broody hens all the time. Slip them under her at night. Sometimes the hen will accept them and sometimes she considers them threats and will kill them. If you are watching when she wakes up, you will be able to tell pretty quickly. Just be prepared to raise them yourself if she doesn't accept them.
Do I have to keep her separate from all the other hens while she is broody?
From your post, I believe you are talking about while she is raising the chicks, not sitting on the eggs. You'll get different opinions on this. If you have room, I believe the chicks develop stronger immunities and better social skills if they are raised with the flock. Mama usually takes care of most of the serious integration issues. Sometimes the other chickens will harm the chicks, but usually mama takes care of her babies and won't let that happen. I think the danger to the chicks from the other hens when thay are raised with the flock is less than the dangers of integrating them later but many people successfully do it the other way. You are dealing with living animals and I cannot predict what your specific chickens will do.