Can I cook the filling in dumplings before freezing?

The filling is meant to be cooked at the same time as the dumpling. That's what happens when you cook the dumpling fresh.

Remember, a freezer is a device for food production convenience. It doesn't really change the basic nature of the food at hand. Rather than make the dumplings fresh and cooking them right away, the freezer just stops time. It's mostly a time machine or storage convenience.

I never do. After making the dumplings, I place on a baking sheet in the freezer and then once frozen put in a container for cooking at a later date.