Substitute 2 leeks for the napa cabbages if desired. I've also made these with 1 head of napa cabbage, 1 head of bok choy, and 1 bunch of scallions. It's a great combo, too.
If you have any dumplings that did not seal completely or that have broken skin, do not freeze them, they'll fall apart later. Just cook those for dinner tonight instead.
To cook frozen dumplings, boil for about 5 minutes before pan-frying in a greased skillet over medium heat until browned on all sides. If the dumplings have not been frozen, skip the boiling step.