As far as NFA goes, scj3081 is correct. He can convert it to a "shotgun" if he prefers.
It's just unlawful to start with a shotgun and go backwards and put a grip on and then a shorter barrel, which is idiotic, but I don't think anyone ever said the government did anything that ever made sense.
With that said, as Ernst mentioned, you can use a smoothbore barrel with slugs no problem. And I'm even looking for a hogue stock in fde for one of my 500's. But I don't know if there is a such thing as an 18 inch 20 gauge 590 barrel. I don't recall ever seeing or hearing of one.
They do make 500C mossbergs in 20 gauge, but as you have noted, the magazine is different, unless you want to change it out to a 500c magazine tube along with the barrel may work.