Preheat oven to 350. Prepare a 13 "x 9" pan with cooking spray and flour; set aside. To prepare batter, combine cake mix, half cup coconut, water, oil, half cup pineapple juice, one 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 half inch 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.