Looking at the picture that Tiffany posted, the only place I see that could fit a wheel is either the bottom near the floor (safest anyway) or on one of the flat sides. If the wire is thin, a wheel mounted up on the wall risks breaking the cage. A wheel, esp. a Flying Saucer, on the floor would leave almost no room for the chin(s) to move.
I LOVE MY Ferret Nation. It was worth every bit of the $200 I paid for it, and if I added lips on the shelves, I wouldn't have half the amount of debris on the floor that I do. I also keep a loose drape of fleece over the cage, in part to corral the stuff and in part to discourage the cat from trying to pet Crash. (She likes to put her paw on other critters and just leave it there. I think it's a comfort thing, but obviously it's not safe for either of them.)