I come from a long line of cabbage roll makers (Ukrainian family!) and the way I have seen it done is the following:
Prepare the cabbage rolls as you normally would if you were going to bake it right away. Roll the rice/meat/whatever in the boiled cabbage, and then put the cabbage rolls into a plastic bag or freezer container. Store the sauce/tomato mixture separately, and then when you go to prepare the rolls, take them out a few hours prior and let them thaw. Then put the thawed rolls into your roaster (or I guess crockpot?) and pour the sauce over and cook. My mom does it that way very successfully!