[Robert]

Can the weapons which can be added to rifles using the "underbarrel weapon" modification in Firing Squad take any modifications or accessories?
What if they are "factory fitted" as e.g. the Yamaha raiden with underbarrel shotgun and grenade launcher?

[Hobbes]

As funny as it would be to have an infinite nesting doll of weapons and weapon mods "My under barrel Grenade launcher has an Under barrel Flamethrower... With a Bayonet!" 

But the answer is no, sadly.