I wouldn't add canned salmon. It is too high in heavy metals and other toxins. Most salmon these days is farm-raised. It is a cruel industry, damages the environment, endangers wild salmon, and on top of all that they are fed corn, antifungals, antibiotics, dyes (to make the meat pink), just like feedlot cattle. If you want to add something healthy to the food, use real meat.
Cats easily become addicted to fish, but it is not really very good for them. Canned salmon is extremely high in ash because it contains bones. Plus, if you're adding anything in large quantity (more than 10% of their total food intake), you have to balance the minerals and vitamins.
If they're refusing to eat unless you add salmon, then they just can't have *any* salmon. It's a serious problem when it affects their appetite. You are the grown-up in the house, you need to do what's right for them, not what they want.