So, either on the Mossberg or the Remington 870 -
can you buy extended magazine tubes?
Choate Machine and Tool can provide extended tubes for many shotguns. I know I added one to a Mossberg 935 tactical semi I got.
Depending on your local jurisdiction as far as attachment, they can be had at a number of sites.
www.cheaperthandirt.com comes to mind for inexpensive parts to hang off your 870/900 guns.
For the Remington 870 an extended magazine is a matter of simply replacing the magazine cap with an extension.
The Mossberg 500 requires the replacement of the entire magazine tube and a new barrel.