Well you could but if you've actually done it you probably wouldn't want to again! It's a big wheel on an Explorer and there's no practical way of getting up and down from the roof.
I had six weeks with a Camel Trophy Disco a few years ago. Spectacularly equipped vehicle - spotlights, foglights, bush wires, bull bar, winch, twin batteries, sand ladders and THREE spare wheels! One on the tailgate and two on the roof but even with the rear ladder, it's still a two man job to get one down without killing anybody nearby!
Tyre Weld anybody?