Now that would be a find indeed - and not just a unicorn - on the order of trophy photos of a pinto patterned unicorn with a sticker or two. I have never actually seen large caliber inserts for a Krieghoff drilling. I have a .22 lr and .22 magnum set fitted for one of my drillings and they work as advertised - the .22 magnum is particularly useful.
There have been some cheap attempts for rifle inserts done here in the States, but to be of any utility, an insert barrel needs finely adjustable base shims (.22 variants) or at the muzzle so that its POI can be adjusted to the POI of the scope and permanent rifle barrel. Perhaps
@HWL will tune in, but I do recall a German company that made larger inserts. Way back in the day, there was a company called "Fritz" or something like that. Looking on the web, Keppler seems the go to company these days. I suspect you will need deep pockets.