Bake cake according to package directions, using a greased 13-in. x 9-in. baking pan. Cool cake completely on a wire rack. In a large bowl, whisk milk and pudding mixes for 2 minutes. Let stand 2 minutes or until soft-set. Fold in pineapple; cover and refrigerate. Fold coconut and nuts into whipped topping.
To assemble, cut the cake into 1-in. cubes; place half in a 5-qt. punch bowl. Top with half of the pudding mixture and half of the whipped topping mixture. Repeat layers. Sprinkle with additional pecans if desired. Cover and refrigerate until serving.