seem to recall there may be an issue as to how long the barrel can be on a "handgun" and when does it become a "rifle", shoulder stocks, and so forth, so on...better check with the local BATFE office for any clarifications.
Hate to see anyone get into any legal hassles,issues, etc.........and the internet is NOT the place for ANY "free" legal advice...............
Yes, any custom shop , gunsmith can make up and install a longer barrel, just need to get, find the cost, quotes, timing, etc.....Brownells, for one, sells barrel 'blanks', need to get it threaded, fitted and finished.........