Ah, the crisp sauerkraut is the best. This is what you do to make pirogy fillings.
Lightly squiz the juice out of the cabbage. If there is some it is not a big problem. Sauté some diced onion in a pan, add sauerkraut, salt and pepper to taste. Cook until the cabbage is soft and is somewhat brownish in color.
Put it into fine mash colander, let it drain. You do not want liquid, otherwise it will sip thru, and the filling will come out during cooking. That is just that simple.
You are what you eat.