Ducklings under a broody hen?
If you have a broody, she will not accept ducklings, just eggs. Try collecting eggs from other females and putting them under her. If she's truly broody, she'll incubate the eggs and give you ducklings of her own. Typically you can't even give chicks to a mother than already has babies of her own, giving them to a broody who doesn't even have chicks will just result in the ducklings getting, shunned, attacked, or even killed!
If you are talking eggs, yes you can put eggs under a broody hen. Just expect your ducklings to be a bit odd. ;) I have 2 hens that were raised by a broody hen and they both cluck. They also quack. It is endlessly amusing.