I'm not sure whether the full game will have this functionality or not, but so far I've not seen any ways to do it. Essentially the per-round top-up functionality of weapons like tube-fed shotguns needs to be in. With the focus on infantry, and notably close-quarter fights, the immediacy and tactical impact of performing these reloads as opposed to the standard mag change makes a whole new dynamics within those envronments when applied to Shotguns, and could give variety to long range marksman weapons.

Even if tube-fed shotguns or non-external mag rifles don't make it to the retail game, this sort of feature is something that the modding community could really do with, and it's absence has been notable for 2 full Arma iterations now.