Well first if your frozen burgers are the kind that are just uncooked frozen patties, thaw them well and you should be back at ground beef stage… then you can pretty much put in whatever you like. For example tonight we’re eating leftovers but tomorrow, provided it’s not raining, we’re grilling burgers with minced onion and jalapeno seasoned with ancho chile powder, vulcan fire salt, and chipotle tabasco.
For something a little simpler (or that at least has easier stuff to find or you’re more likely to have) try mixing in minced onion, garlic, pepper (hot or mild), along with salt (smoked if it’s available) and fresh ground black pepper. Also, you can micorwave some bacon until it’s nice and crisp then break it up into it.
If your frozen burgers aren’t just uncooked preformed patties, rather than mixing stuff into the meat try frying up onions, garlic, and peppers mixed with some salt then make your burger grind some pepper on to it, cover with the stuff you fried, and top with some shredded cheese. I sometimes make smoked bacon mayo to put on burgers, but you can get a similar effect with a couple slices of crisp bacon crumbled into a tablespoon or two of store mayo. Toasting the roll can make a difference too.
You can also season the oil the burger is going to fry in and cook it slowly with a lid on the pan, and season it mid way through that should allow more of the flavor to get into the burger.