PREHEAT oven to 375°F. Remove 2 Tbsp. of the sugar; set aside for later use. Mix the remaining sugar and the tapioca in large saucepan; stir in water. Bring to boil on medium-high heat. Add plums; stir gently. Cook 2 min., stirring frequently. Spoon into lightly greased 8-inch square baking dish.
COMBINE baking mix, reserved 2 Tbsp. sugar and the cinnamon in medium bowl; stir in milk just until mixture forms soft dough. Drop by heaping tablespoonfuls onto hot fruit mixture.
BAKE 28 to 30 min. or until biscuit topping is golden brown.