For an SLR, such a device would have to mount on the front of the lens rather than between the lens and the camera body. Such things do exist -- in fact, I own an FC-E8 fisheye adapter for a Nikon Coolpix 995 and a 0.5x Barlow lens for my microscope. If you search eBay for something like "0.45x wide angle lens," you'll also see several generic examples which purport to work on Nikon dSLR's.
But for an interchangeable lens camera, it's typically less expensive to design and build a wide-angle lens rather than a screw-in wide angle adapter of the same quality.
"There is no real magic in photography, just the sloppy intersection of physics and art." — Kirk Tuck