The problem is that the chicks have to accept the hen as their mother too. She might accept them but if they're older than a few day, they won't see her as a mother.
I honestly didn't even slide them under her. It was broad daylight and I set them out in front of her with the other chicks she had. She gave each one a single peck, probably either a 'Well where have you been til now?' Or a 'Get over here where it's safe from that person.'
I gave my girl 4 turken chicks after she hatched 4 eggs. I took them down one at a time a few hours apart and she took each one. It ended up one in afternoon, one in evening, one in morning, one in afternoon
Most people say to wait until about 14 days so it shouldn't matter it's a week early. And I would take the egg away. If you're willing to risk it, you can always try to give her two or three and then take the egg away.
I would personally wait a few more days before giving her babies. She should still continue to brood on the fake egg so she will still be waiting for one to hatch