I forgot to answer your first question. There is no way to add a magazine extension to the Mossberg 500 or Mav 88. The Mossberg 590 will take an extension due to the different design of the magazine/barrel locking system. If you turn the screw/bolt at the end of magazine you can take the barrel off (pull back forend halfway first). You can see that the part the bolt screws into closes off the magazine tube. Other shotguns use a larger cap type screw on the end of an open tube, which is why they can be extended.
The only way to add rounds is a side saddle or butt cuff. But this adds weight and bulk and some of the things that makes the 5+1 Maverick 88 great is the short length, light weight, and slim profile. Yes, I have other shotguns that hold 8 rounds but they don't have the maneuverability of the 18" Mossberg.
And since you violated the unwritten rule of posting pictures of your new gun, I'll help you out