If not, why not?
I've seen very small, quiet gasoline generators. I would think this could solve this problem easily at the expense of a small amount of storage space.
You don't even save much by keeping the gas tank mostly empty when you aren't expecting to need it -- the gas weighs ~6 lbs / gallon, so you might save 50 lbs by keeping the tank on 1/4 when you aren't planning to drive past the battery range.
Think about it this way: batteries are really heavy, and the Tesla has like 5x more battery capacity than a Volt. Meanwhile, if you're mostly making short trips, you're hauling around 3/4 of those heavy, heavy batteries as a contingency.
Even if the generator was in a small trailer, that would provide the range freedom without loosing the storage space (and help with venting/cooling/noise).
This has been done; but the small trailer also adds significant friction and aerodynamic drag. It also adds complexity in driving and maintenance.
As others have said, it's more practical to have a purpose built hybrid.
From a regulatory standpoint, yes, fuel systems would be regulated in this design.