You could always "rifle" it yourself... Just like the good ole days. There are numerous videos on YouTube about doing it to any barrel.
Other than that... I've never seen anything about a rifles barrel for the KSG.
I just buy rifled slugs. If you don't know, the slug itself is rifled. They are used in barrels without rifling.
They work great.
I would research the comparison of the rifling vs. non rifling on shotguns. Shotgun barrels are substantially thinner than a rifle barrel. Make sure scarring of the barrel wouldn't occur from buckshot, etc. If you did decide to possibly rifle it yourself.
I would think that swapping barrels to a rifled one would cause functionality changes if the barrel thickness was increased.