To extend the berth aft requires cutting out the bulkhead between the main cabin and the starboard sail locker and building new bulkheads and a bottom to enclose the space. The bulkhead between the main cabin and sail locker is non-structural and about 3/16" thick. I would also shorten the berth from 7' to 6' 8" to give the new locker up to 24 inches of width and to keep the berth from extending past the forward end of the sail locker hatch in the cockpit. The battery could be located below the berth extension in about the same place as the OEM setup. A new battery box would need to be built.

The locker would be built from 1/2" plywood and could be faced with teak. It would be easy to build it as an add-in without cutting any of the OEM parts. It would just be bolted on top of the settee bottom. This would allow for restoration of the OEM configuration. The top of the locker could go to the cabin ceiling or stop even with the underside of the side deck providing a small shelf area. This would be a lot like the locker at the front of the starboard berth on the Pearson Ariel which you can see in the photo below. The new locker would have a volume of about 9 cubic feet, a bit more if you include the space taken from the OEM shelf outboard of the settee.