Gently mix the shrimp, imitation crabmeat, cabbage, celery, and green onions together in a large bowl. Stir the mayonnaise and sugar together in a separate bowl until the sugar is dissolved into the mayonnaise; add to the shrimp mixture and gently stir until all ingredients are evenly coated.
Refrigerate for 3 hours to allow the cabbage to soften and the flavors to blend.
