Yes, you can add tuna to seafood macaroni salad. If you are using canned tuna, make sure to drain it well before adding it to the salad. You can also use fresh tuna, but it will need to be cooked before adding it to the salad. Sear the tuna in a pan over medium-high heat for about 1 minute per side, or until it is browned and cooked through. Then, chop it into small cubes and add it to the salad.