There are 2 factory approaches you can follow to add a single seat.

You can install a single person center seat that was optional in just a
few years 85/86, sliding onto standard floor tracks that can be fitted
to any year van using the threaded holes always present in the floor.
These have a lockable storage compartment in the front of the base and a
small open area in the rear of the base. These also swivel 360 degrees.
The seatbelts attach to the base and the whole setup can be taken in and
out at will, though it is heavy. These came in tan fabric only but any
Vanagon front seat will slide onto the metal base/swivel section. I just
put one of these in my pass van a few days ago.

The other factory choice is the pass side rear facing jumpseat using the
factory parts from an 86+ Wolfsburg pass van, Carat pass van, or
Multivan. It is not that hard to do but safety is involved so the
attachment points on the floor are important to get right. If you have a
spare seat with matching fabric to your other seats, you can disassemble
the jumpseat and put the upholstered sections of your spare seat on the
jumpseat frame, in some cases.

Mark