Yes. You do not need to defrost them. You may directly put frozen meatballs in the oven but take care of a few things. Spread them apart and double the time to bake. Bake them for 25 minutes instead of 12. This means, as per this recipe, you do not add the sauce until it is half-done i.e., 13 minutes. Then, add the sauce and follow the recipe from here on.