No not spices but toppers. Popular toppers include crushed up treats, a sprinkle of FortiFlora, the liquid from a can of tuna, a squeeze of those new lickable cat treats etc.
If your cats will eat their food without issue, toppers aren't necessary. Some cats get addicted to food with toppers that they will not eat without the topper and will go on a hunger strike. Limit toppers to an occasional treat.
What do you feed your cats? Sometimes cats get bored with their food. If you feed only dry, include some canned food a few times a week.
My kitty likes nutritional yeast. When she balked at eating her second dose of Metronidazole mixed in her food, a sprinkle of nutritional yeast got her to eat half of it (not the whole thing, sadly).
There is also bone broth to consider - pet stores have it or you can make your own (which is probably better in the long run). That works well to moisten canned food that has gotten a bit dry, however if water suffices as a moistener, then as LTS3
said no real reason to introduce toppers just for the sake of it - other than as an occasional treat. Another one is bonito (dried fish) flakes, a lot of members say their cats like them. I tried them with Feeby, and that was a definite 'no go'. So, you never know...