I have no experience with ducks, but I have had that work with chickens. The age difference makes it tough and not all chickens will accept new chicks. It's not something I'd try with more than 5-7 days difference in the chicks, either.
When I try it, I like to make sure the new chicks are steady on their feet, because that hen is going to be on the go with her older chicks. I also slip the chick under the hen when she's back on her nest at dusk or dark. Then I check a few times to make sure things are going well. Be aware that if they don't go well the hen will kill the chicks in no time at all.
Congratulations on your hatch & good luck!