In a large resealable plastic bag, combine the dressing, sugar and seasonings. Add the fish; seal bag and turn to coat. Refrigerate for 10-30 minutes.
Drain and discard marinade; place fillets on a broiler pan coated with cooking spray. Broil 3-4 in. from the heat for 10-12 minutes or until fish flakes easily with a fork. Place 1 teaspoon butter on each fillet; let stand until melted.