Three and four-wheel all-terrain vehicles (ATV) cannot be upgraded for on-road use. ATV's are manufactured with a live axle, rather than a differential gear, which seriously affects their handling characteristics when operated on paved surfaces.
I think this is what has been challenged from what I have heard up here. Not all ATV's have a live axle. I have heard, but can't confirm it, that if it is shaft driven and has a diff then you can get plates for it providing you get the turn signals and mirrors.