Check your local laws, you may not be able to add extra seating in the truck since it didnt come with extra seats from the factory.
That being said, should it be legal. E.G. Here where i live it can be done, but you have to go through inspection and reregister your vehicle as a Ubuilt...which changes insurance,etc. Anyways, if it is legal.... What are you using them for? are you thinking for children? This leads to issues with local laws again, especially car seat laws. There isn't much room back there for them, but you could use jumpseats from an older ranger. I built a bench seat for an old 91 ranger I had, my advice is forget it....my children at the time were 2 and 3, they fit, but with very little room since I have to have the seats back ( I'm 6'2" ). For adults, forget it, even in jump seats, I fit in them, BUT in an accident I'd be completely screwed.
Keep in mind that my measurements are based on a 91 Ranger, ext cab. So if your cab is different they may fit. You could also try the fold down seats from an explorer. I dont know the exact measurements of them, I could measure the ones in my explorer in the morning if you want. These would fold down, etc and give you your cargo space when needed. I dont think you'd fit an adult back there though.
I'm not saying its not doable, you may need to modify your cab a little more than you may want, After I did away with my 91 Ranger, I came up with idea of adding a pan the the back of the cab and sinking the seats back an extra 2 or 3 inches, for that added leg room, it doesnt seem like a lot, but it would feel like a mile.
Another option would be to get a bench seat from a dodge dakota. They may fit better with less mods.
Just my thaughts...