well they do use bean bag amunition for shotguns and the japanese did use explosive rounds in there rifles during ww2 so I would imagine you could rig up exploding sabos for the automatic Franchi spaz 12, 15 or a benelli shotguns.

Besides the Spas 15 is boxfed and semi auto so there goes all the problems with using a shotgun for a zombie invasion to begin with, and for the others just use a speed strip.
http://www.botachtactical.com/ccws.html