Couple of points to bear in mind for your project:
Sleeving with damascus tubes is very possible, I have seen a couple of examples and, although the joint is rather visible if looked for, a passing glance will probably miss it if you are lucky with the match in pattern.
As a converted pinfire, you may well find that the wall thickness at the breech and on through the chambers is considerably less than a more recent centre fire barrel. This means that there is less thickness to accommodate the old breech piece and the internal top-hat of the new barrel.
This thinness also could compromise the safety of the barrel in the chamber area, not a good idea.
One option might be to use 16b tubes sleeved into the 12b stubs. These might more closely match the external dimensions of your old 12b pinfire barrels.
Lastly, extractor holes can run very close to the chamber internal surfaces and might make additional welding necessary on the donor barrels after machining the top-hat.
I would suggest that having a good set of damascus tubes re-lumped to fit your gun may be a neater solution and possibly no more expensive, if you can find someone with the skills to do it.
