How can I add flavor to meatballs which taste stale from being frozen too long?

Whenever I have freezer burnt or really old frozen meat, my go-to solution is to use my slow cooker. Find a recipe that sounds appetizing to you and has a long cooking time (6-9+ hours on Low), and go for it. For example, here's a Meatball and Vegetable Soup recipe (which I'm not endorsing, it's purely as a "this is the style you're looking for").

I've never had much success using old frozen meatballs (or other frozen ground meat products) in their original shape. I agree with mech though that a slow cooker recipe infuses flavor and restores tenderness. If you don't have one, or aren't in the mood to use it, there's another way to achieve the same result. Choose the recipe you want, put all the ingredients together with your meatballs into a tall pot. Cover it, put it on the stove and simmer at a low level heat for a few hours.

As for me, I generally mash old meatballs up, just like you did with the sloppy joes. Then I treat them as I would fresh meat, and add whatever seasonings I'm in the mood for. I don't have a very adventurous palate, so my go-to choices are usually mild, like onion powder, garlic powder, oregano, parsley and basil. Some of my friends use cumin, ginger, curry or other types of spices.

Another thing I do with mashed meat is add a few fresh ingredients. (You may have done this with your sloppy joes, as I haven't seen your recipe.) For instance, sautè some chopped onions, peppers, mushrooms, garlic, and whatever else you like, then mix in the meat. The new ingredients do a decent job of making an older product taste fresh. I also like cheese in everything, so depending on the preparation, I'll add grated cheese to the pan or baking dish, or sprinkle some on the finished dish. I like aged parmigiano-reggiano or asiago, but there are spicier cheeses that also help mask the flavor of stale meat.

The stale taste is because when food is frozen for too long, it can dry out. You want to add egg to the meat to moisten it. Make sure to blend the whole egg into the meat so that it can be thoroughly absorbed.