I doubt it has 22" tires/rims - more likely the rims are 26" or 29" (in which case it should be listed as a Rockhopper 29) and the tires are 2.2" (and the seller forgot the decimal point).
22 inch wheels on bicycles are pretty much a botique item for BMX, and certainly wouldn't be found on an adult sized bicycle (and the kids bikes from Specialized are sold as the Hotrock in sizes 12, 16, 20, 24). For general reference, there are some juvenile bikes with 22" tires, but the Rockhopper is not one of them.
Also, in general, switching the rim size is difficult with rim brakes (since the rim brakes have to line up - one special case is 27" to 700c which requires a 4mm brake adjustment), but sometimes doable with disc brakes (700c -> 650b is not uncommon) provided the setup clears. Its generally a ymmv/you're on your own option.