You can animate draw order by selecting the "Draw Order" category. It is divided into two parts, one for the start draw order and one for the current draw order. The start draw order is set before frame 0. Then by using actions like "After", "Before" and "Inside", a new draw order is calculated giving the current draw order.
In the bottom we got a button named "Change". Use this to add the draw order action at the selected frame.
