It will be alot easier to just buy the 18.5" model you want and sell the 20" you have. The only place that I can think of that might have just the barrel would be AIMPRO. They will also do the conversion for you but it will probably cost the same as just buying another shotgun.
BTW, the adjustable stock on that 590 is ATI crap.
"Glocks are for criminals" -Nick_45