Are you saying you would want a wall type structure? As in you upgrade it into a stronger, more worthy wall? Or do you mean an actual destructible. If you mean that, all you have to do, is go in, make a new trigger, go to configure action, and find the function in the destructible category that says Make Vulnerable/Invulnerable (Last Created Destructible). Then you can change the (Last Created Destructible) to make it a certain selected destructible. You can repeat that action as many times as you want to select all of these "rocks".