I wouldn't even think about it. I'd be too afraid to ruin the rifling and that cannot easily be repaired, as far as I know. I've noticed that in the past, both CVA and Thompson Center either sold dual barrel kits (1 smoothbore barrel and 1 rifled barrel to be used with the same stock and lock) or otherwise made available a second drop-in barrel in smoothbore separately to go along with the rifle barrel sold initially with the rifle.
Part of this may have been simply to enhance existing sales, but I'm sure this was not the main reason. If it was safe to shoot lead shot in their rifled barrels I'm sure they would've used that as an advertising advantage to move their products.
Another thing to point out; several years ago 1 or more of the U.S. manufacturers sold a .22 smoothbore. It was simply a .22 rifle barrel without the rifling. This being a modern smokeless powder weapon, I'm sure this was developed for a reason and today you can still find the rimfire .22 "shot" cartridges for these little smoothbores.
My point is, if these manufacturers went to all the trouble to develop and market a rimfire .22 smoothbore I'm sure there was a
functional reason for it beyond merely seeking enhanced profits.
I'd simply tool up a smoothbore barrel to compliment my rifle barrel and use that on the gobblers.