I know Kreighoff does. They make barrels of different calibers and a 20 gauge shotgun barrel for their receivers.
The concept of bringing this into Africa has been gone over here before. I don't remember the outcome. I believe if all the attachments had the same serial number it's one gun. Different serial numbers make different guns.
My 470 and 9.3x74R barrels have the same serial number as the reciever. The 7x65R was added later and has a different serial number. I imagine when I add a shotgun barrel set it too will have a different serial number.

When you add barrel sets after the gun has been made it can only be of a smaller caliber than the gun was proofed for. Otherwise the gun has to go back to the factory for proofing of the new barrel set.