You do not need to thaw meatballs beforehand; you can simply add them directly to the crockpot or slow cooker.
Grape Jelly Meatballs
4 dozen fully cooked frozen homemade meatballs or 1 (32-ounce) bag frozen fully cooked homemade-style meatballs
24 ounces jar chili sauce or tomato-based barbecue sauce
1 (18-ounce) jar grape jelly
Add the frozen meatballs to a crockpot. Add the chili sauce (or barbecue sauce) and grape jelly. Cover and cook for 2 to 3 hours on high or 4 to 6 hours on low until the sauce is hot, and the internal temperature of the meatballs reaches 155ºF.
Serve directly from the crockpot with a serving spoon or toothpicks, or transfer to a serving dish. Alternatively, you can cook the frozen meatballs in a saucepan. Add the chili sauce (or barbecue sauce) and grape jelly. Cover and simmer over medium heat until the sauce is hot, about 10 minutes.