You've hit on one of my pet projects! I have been into pistol caliber carbines for a number of years. They are alot of fun and there is a practicality to 'two guns one box of ammo' no doubt. The Handi is available in a couple of hand gun calibers, but only revolver cartridges for whatever reason. Why they don't offer guns in the auto pistol calibers makes no sense to me. I have a couple of bolt actions (45acp, 9mm) and now I want to get single shots in all my favorite pistol calibers. While it's true enough that a 38 spl is near enough to the 9mm, I have both in handguns so why not get rifles for both. And a few more....
For a 9mm barrel, I am looking for a 20 or a 16g shotgun barrel. That leaves me plenty of room to fit a stub, as well described in the posts alluded to in this thread already. You can get (direct from NEF/H&R) a .223 extractor that will work with fitting on the 9mm case. If you can DIY, you will be in and out of the project for a little money and a bit of time. If you find a smith to do it, you may well pay more for the barrel than you paid for the gun to begin with, but if it's what you want just do it. It's your gun, your money, your choise.
Definitley read the stubbing article in the FAQ. You can decide for yourself if it's a project you want to tackle yourself. Reading what's involved will also help you speak knowlegably with a gunsmith about having work done. You may want to take a partial DIY aproach. Have a gunsmith or machine shop do the lathe work, and you fit it up.
I will make a final warning about stubbing. Be careful to NOT MAKE a short barreled shotgun, EVEN IF ONLY FOR A LITTLE WHILE. If the machinist has the barrel, but the frame is not in his shop while the barrel is cut down, that should be OK. Once the stub is threaded or otherwise bored out so a shotshell won't fit in the stub, you should be OK. You just don't want to cut the barrel down, then leave it to put the 9mm blank into it later. That's a big no-no. Not hard to work within the law, just think ahead with an eye toward staying in bounds.