I have no doubt that it can be done, but I'm wondering if it'd be worth the effort in terms of being able to hit much of anything with it. Being widely known for having a perhaps excessively high regard for the well-being of my Personal Favorite Body Parts, I'd have to admit to some safety concerns, too.
Seems to me that the bore and cylinder throats would be grossly oversized for the 0.451-0.452" bullets on modern .45 LC ammo. One could likely buy lead slugs of 0.454", or cast them however large you'd like but wouldn't you have some problems getting them to fully chamber in a lot of .45 LCs?
I'd be real wary of touching-off a full-strength BP cartridge load, much less a modern smokeless .45 LC load, in any cylinder that hadn't been properly heat treated as part of the conversion process.
Before we get into the whole "standard charge for the Walker was 50+ gr., so it could surely stand up to 38 grs. in a cartridge", "modern steel is so much better that it doesn't matter" spiel I'd like to respectfully point out that those 50+ gr. charges were under a RB that weighed about half of what the standard .45 LC conical weighs. And that many replicas are made from mild steel which has had only slight cosmetic case hardening on the frame or hammer if it's had any at all.
At least the commercially made conversion cylinders for the Remmies get that heat treat. And the factory replica Richards, R&M, and 1872s aren't made from the same parts used in C&B models, plus they're chambered for much-less powerfull cartridges. Does anybody even make commercial smokeless loads for .38 or .44 Colt? FWIW, the only solid info that I can find on custom-made conversions on replica C&B models include a new, specially fabricated cylinder as part of the cost. So far, I have found only one who starts with the stock cylinder, and he sends it out for special heat treating post conversion.
YMMV. Personally, while I might be curious to see such a conversion, I'd have to have some major faith in the engineering and machining expertise of the maker before I'd fire one myself.