Without a doubt and probably impossible anyway, if you want to replicate OEM methods.So likely to be more expensive to buy the equipment from VW and retro fit in to a 2 berth GC600 than to just pay the extra for a 4 berth.
There seem to be more 2 berth 600s than 4 berth versions for sale at the moment (esp for a reasonable price)
The VW California Club is the worlds largest resource for all owners and enthusiasts of VW California campervans.