Sure you can.
Get out the sawzall and cut off the rear roof of the van just behind the driver seat. Take the sides down so they are no more than around 18-24" high.
Remove any seats or other structures in the back and then mount a 5th wheel hitch over the rear axle. You'll probably have to custom fabricate the attachments as the standard ones are designed for pickups.
By the time you are done, you probably could have bought a suitable used pickup and not trashed the van for a whole lot less effort.
Somehow fabricating an extension aft of the receiver hitch...that's a non-starter for a host of reasons.