You're SOL unless someone has a barrel they want to sell and someone that can safely cut it off and weld in on, no one makes underlugs, although they have been made before, very expensive machine work, might as well spend the money on an accessory barrel or a used 10ga. Be aware the 10ga requires a different frame than other shotgun barrels, it won't fit an SB1 or SB2 rifle frame, so unless you have an SB2 shotgun frame, you'll need that too.
Tim