I can't find where to get either a magazine extension or barrels for a Fabarms FP6. All I do know is that HK stopped listing their stuff on the website.
Not positive what you have exactly, but my friend has this HK shotgun that has 'Accokeek MD' on the side, which happens to be the import point for Benelli and Beretta shotguns. I'd give them a call.
Benelli USA
17603 Indian Head Hwy
Accokeek, MD 20607
P: 301.283.6981
F: 301.283.6988