Sorta... But any addition must be rifled; the rifled portion must be of the required length.

My 10-5 came with a bit of barrel skilfully soldered on. Presumably the 'smith had shortened a barrel and had the end of a barrel so he aligned the grooves and silver-soldered it on. Barely noticeable from the outside, aside from visuallly moving the front sight back a bit. Looking at the join on the inside I thought I could detect a bit of ss in the grooves. It shot OK, but my marksmanship isn't the best, so I decided to get the 5" barrel in the pic above. I'd imagine the 'smith's bill was fairly high as that's not a simple bit of work.

This would have presumably been done sometime after about 1996, when the idiot Bill C-68 made a nice 4" Model 10 into a Category 12(6), ie "Prohibited", and would have required the owner at the time to re-register it as such. Those who did so were grandfathered in, so they can sell to others with the same licence, but cannot buy a new gun that falls into that category. (And none were allowed to be imported into Canada after that anyway.) I missed out on all that, so I'm pooched But as someone posted above, this is why US gunmakers began making handguns with barrels that just meet the required length.