The raiden never had an internal underbarrel mounted weapon in previous editions so that would be new to SR6.
As to a combination shotgun/grenade.... no.
Shotguns are .75 to .85 caliber (under an inch diameter). The smallest propelled grenade was 25mm or 1 inch. (Never actually deployed. Too small a charge for any real damage) While the "standard" propelled grenade is 40mm or 1.5 inch diameter.
Without a properly sealed breech, your "gun" is now a bomb, with 30,000 psi to spray shattered metal into your face, arms and torso.
Choose which one you get at purchase. (Or buy 2 one with each!)