The idea was that he could still come in and out through the small door(s) into the playpen, but it would be attached to the cage front (at each corner on the front) rather than all the way round. He could still climb the front of the cage and would need something across the top to stop him getting onto the top of the cage and escaping.
This picture may help with my idea.
the wood hutch would be Charlie's cage and the wire run would be where the playpen goes.