Some things that might work:
Plain yogurt. I mix a tiny bit with the food. Sometimes I just drizzle the liquid that stays at the top when you buy the big containers of yogurt.
Cheese. It can be sprinkled throughout the food. Don't just dump it on top otherwise the cheese will be the only thing that gets eaten
Fish oil. My dog goes CRAZY for Grizzly's Salmon Oil. I use this a few times a week and just put half a pump on the kibble and then shake the bowl around so the oil covers all the kibble.
Chicken broth. I use a low sodium chicken broth and just drizzle it over. I'm sneaky and I dilute it with water so it's really just like 1/4 broth and 3/4 water that my dog gets in her food.
Canned food. Sometimes I mix in canned food with the kibble. It's just annoying and smelly so I don't do it often. My dog loves it though!
Eggs. If I do this I don't mix in a ton, just enough so the scent/flavor is there.
Hope one of these suggestions works! I know how frustrating a picky eater can be!