AbitNutz, such nipple conversions are done mostly in the UK, I even remember a Remington 1858 cylinder made specifically to fire smokeless powder - it had stepped chambers & 209 nipples.
This link is for one of the several UK gunsmiths that offer ROA nipples for shotgun primers: http://www.anvilconversions.co.uk/index_files/Page13768.htm
That's kind of nifty:
Edited to add they sell nipple conversion for Pietta's and Uberti's.