Place berries in 6-quart slow cooker; stir in sugar and corn starch to coat berries.  Note:  frozen berries do not need to be thawed.

Combine biscuit mix, brown sugar, oil and milk in a bowl, stirring well to make a soft dough.  Spread dough evenly over berry mixure.
 
Cover slow cooker with a double layer of paper towels, then place lid on slow cooker (over paper towels).  Cook for 3 to 4 hours on HIGH, until topping is puffed and center springs back when touched. 

If desired, sprinkle cinnamon sugar on the biscuit topping just before serving.