Generic underbarrel grenade launcher to install on any weapon, instead of the specific underbarrels for specific guns?
What you want is the "Underbarrel Weapon" mod (Page 153 of Arsenal, table entry on page 151 of Arsenal).
So it goes like this:
Pick your weapon (Say, AK-97), Arsenal recommends restricting the base weapon to a longarm, machine gun, or assault cannon. But, it's a recommendation, not a rule and Assault rifles are just as good a choice, IMHO.
Add the Underbarrel Weapon mod to it (cost = weapon added, Availability = weapon added), using up 3 slots and requiring a facility to install.
You, the GM then approve whatever weapon he wants to install as the underbarrel weapon. For grenade launchers onto anything smaller than an LMG, I recommend the ArmTech MGL-6 (Arsenal, page 31). This is a "pistol" grenade launcher, so size-wise, already close to real underbarrel launchers.
Modify as follows: Ammo capacity halved (using the MGL-6, that gives you ammo capacity 3) and it loses all existing recoil compensation. If the base gun has underbarrel based RC (underbarrel weight, foregrip, biopod, tripod) itself, this has to be stripped out before installing the underbarrel weapon.
Personally, I'd argue that any RC that is "generic" on the base gun (sling, folding stock, for example) would apply to the underbarrel gun, while RC "specific" to the base gun (gas-vent system, electronic firing) would not. But, this is a GM's call, the book is silent on the question.