PF:
I've seen many older drillings with the rifles barrels already sleaved to some small bore caliber (22 of some sort, or an old Western caliber like 32 something). I bet a lot of war trophies came back to the US and metric rimmed ammunition was not availible so they had the rifle barrels sleaved. They should not be too hard to find. In fact I have the opposite problem I'd like an older drilling but with the larger rifle caliber and many of the ones I've seen were sleaved!! Take a look at Gunsamerica.

Edit:

Pheasantfinder I am sorry I misunderstood your post abit. You are asking about sleeving a drilling, rather than finding one with a sleeved barrel. Oops. Sure its possible. Though my inclination is to leave the existing drillings in their original caliber, but to each their own with their own guns. Have you thought of a combination gun? I've seen some with 222 rifle barrels if memory serves me correctly. Just a thought.