Not really practical without major surgery and engineering. The top is defined by the main fuselage longeron, and the bottom is defined by the fuselage floor. Can't extend it forward without messing with the bulkhead where the rear spar attaches and the rear seats, and enlarging it aft gets into the bulkhead between the tail cone and passenger compartment.
Can it be done? Sure. But probably not the kind of thing for bubba the engineer to SWAG.
What are you trying to accomplish? Is there something you want to put back there that won't fit through the baggage door, but could fit through the doors and past the folded down back seats?
