I made a congealed salad & it didn't set well.. Can I add plain gelatin? If so, do I sprinkle the powder directly over it or should I

Could the entire salad stand to be warmed up? I'm thinking bloom the plain gelatin in cold water while you heat the salad. Once the gelatin in the salad melts, stir in the plain gelatin until it dissolves.

Might even work if the reason it didn't set properly was due to fresh pineapple…

It doesn't sound very salvageable. If you're going to try, I'd dissolve the gelatin, and mix it through the salad.