You could be between a rock and a hard place. First, measure the barrel the way it is done for legal purposes. With the bolt closed, insert a rod into front of the barrel until it hits the bolt face. Mark the rod just at the end of the barrel, remove the rod and measure from its end to the mark. If that distance is less than 18", the gun is illegal unless registered as a short barrel shotgun. If it is 18" or more, OK.
Now, if the barrel is less than 18", and if the gun is not registered as a short barrel shotgun, it is illegal under Federal law and possibly also under state or local law. Since the law has already been violated, you can't really do anything legally except to surrender the gun to BATFE or the police.
Someone in that situation might have the idea of welding on a choke or extension, but no gunsmith would touch the job since the gun is illegal.
Of course if that someone were to destroy the barrel, cutting it into pieces and scrapping them, then the gun would be simply a gun without a barrel. Then that someone could order a new barrel. Gun Parts Corp. (
www.gunpartscorp.com) lists 20" cylinder barrels, 12 gauge, for a little less than $200. That could be what someone might want for home defense.
Jim