We have a black copper maran raising her own natural chic. We now have Day Old Incubated Chics: (naked necked-polish mix) can they go back to the coop and have the copper maran raise them? Or do they have to stay in brooder box setup.
You can sneak them under her (best to do it so she doesn't realize it) in your sleeve since they are day olds. Here is the really really important thing: IF she pecks at them when they come out from under her to get food and water, get them out of there. She may tolerate them under her, but wait and watch for them to emerge.
You can try to sneak the chicks under your black copper maran hen but you have to watch and see if she attacks them.
If you are going to sneak them in, it's best done at night so she will stay on them all night, listening to and becoming familiar with their little peeps and movements and the chick she already has can accept them too. Then their new mum can wake in the morning and think what a clever girl she's been! As mentioned though, you need to be there when they wake to ensure she has accepted them and doesn't attack as it will take no more than a few seconds for her to kill them if she feels so inclined.....and eating or part eating them would not be out of the question for her either.