Preheat oven to 350. Prepare a 13 x 9" pan with cooking spray and flour; set aside. To prepare batter, combine cake mix, 1/2 cup coconut, water, oil, 1/2 cup pineapple juice, 1 teaspoon rum extract, and egg whites. Mix well. Pour into prepared pan. Bake for 30 minutes or until lightly browned on top. Meanwhile, combine remaining pineapple juice and sugar in a saucepan. Bring to a boil. Using a fork, prick cooled cake at 1/2" intervals. Pour hot pineapple mixture over cake. Cool completely. To prepare frosting, combine frosting and remaining rum extract. Mix well. Frost cake with mixture and sprinkle with remaining coconut.