Pressurized 337's
 

So my understanding is the P 337's only have 5 seats, as the FAA wouldn't certify them for 6 seats without an extra door. Can you add an extra seat as an after market thing?

I saw a listing the other day for one that was for a 6 seat pressurized model and it has me curious if that's allowed.

Other than Regs, there's nothing stopping you from adding a sixth seat to a P-model. They use a different rear seat than the Non-P models, and each side (L/R) is unique due to the location of the outflow and safety valves on the rear bulkhead between the seatbacks. You would likely have to have the extra seat(s) made for you, but they are very simple seats, nothing fancy at all. They don't even use tracks, they sit on the floor. The seat belt when being worn holds the seat in place.

That plane does not have six seats, it has four. The seller is the owner's daughter who knows zero about airplanes. But no it would not be allowed, and an insurance company could deny a claim based upon it.