Yep, you could absolutely -and easily- swap in an 18" barrel. You can find 18" barrels frequently on GunBroker or eBay. I'd even consider selling the one I've got to replace it for one with rifle sights.
I'm not sure about your other question, though. This model shoots 2 3/4" or 3" shells.
I've done a few upgrades since the original post, too:
-OEM 2-shot Extension
-Scattergun Technologies +2 spring
-S&J Hardware delrin follower
-S&J Hardware jumbo head safey (with red automotive touch-up paint added to the Fire side)
-S&J Hardware detachable shell carrier
-M1 Garand-type cotton web sling
-Talon QR sling swivels
Likely future upgrades: 18" barrel with rifle sights - upgraded with tritium sights, and a flashlight.
photo upload
Let us speak courteously, deal fairly, and keep ourselves armed and ready. - T.R.