You can absolutely use regular (unfrozen) meatballs instead of frozen ones!
For Pre-Cooked Regular Meatballs (Not Frozen):
You can go ahead and use them as directed in the recipe. Since they’re not frozen, they’ll likely be ready faster. I’d recommend checking them around 1.5–2 hours on high or 3–4 hours on low to avoid overcooking.

For Raw Regular Meatballs (Homemade style):
Brown the Meatballs: First, brown the raw meatballs in a skillet over medium-high heat. This will help keep them from falling apart and will add flavor.
Slow Cooker Time: Add them to the slow cooker with the sauce mixture and cook on high for about 3–4 hours or on low for 5–6 hours.