I doubt it. If a wild mallard is the mother, they're wild ducks no matter who the father is. If a mallard drake mates with one of your female pekins, though, the babies are yours because the mother is yours.
That said, you might want to check with your state's wildlife and agriculture bureaus. Or maybe contact the federal Department of the Interior to be sure. It's possible that they'll let you keep the mallard-pekin blends because they are suited to be able to survive in the wild. Best to check to be sure, though. You don't want to risk getting hit with fines/prosecution for "poaching."