Just a bit of trivia - as I would never encourage anyone to do something illegal...
Due to different rules around the world, and the proximity of GMRS band to VHF - a lot of VHF radios are designed to be able to be used on GMRS (and FMS) frequencies. These are locked out on radio's intended for US sales. Some radio's use software lockouts - which can sometimes be easy to worked around. Other's have a one or two components on board that cause the lock out - which can be removed with a snip of a wire.
To add to all of that, there are a couple of other bands that can be legal to use in very specific circumstances, so it is technically not illegal to perform the mods that open the radio's up to the additional bands (such as a MARS mod) - you can even find companies that will do the mods for you (on newly purchased radios or at ham meets). Now don't get me wrong, it is still illegal to use them outside the HAM bands (unless in very specific circumstances), but not illegal to have them ready to use...
For example, I might have a couple handhelds that can easily transmit on UHF, VHF and any FMS or GMRS frequency I might want to - or not. LIkewise, my mobile Ham radio can easily listen to those frequencies (even though it is not modified to transmit on them-have not bothered to have the modifications done).
—-
All that said, it is probably just cheaper and more legit to get a GMRS radio to start with.