What you want is a Rollex 20 folding camera. It was a piece of junk from US Camera Company, but it had two very important features. First, it had knobs on both ends - you can turn the film in either direction. Most folding cameras had a knob on only one side - since you don't rewind roll film. Second, you can load 120 or 620 film in it - this camera will digest either one with no mods.
Put your 120 in one side, and roll it onto a 120 reel. Of course, you don't take any photos, you just wind.
Then, open the back and put a 620 reel where the 120 fresh roll was before. Close the back and rewind the film by rolling the other way.
You now have a roll of 120 on a 620 reel. Take your photos with your 620 camera.
If you have film processed, get the reel back, they're a bit scarce. If you do your own B&W, no biggie.
How do you get 620 reels? Buy a couple of 620 el cheapo cameras on eBoy. They usually have one in them. You can always ask to be sure.
Here's a link to a Rollex 20:
http://cgi.ebay.com/Rollex-20-Foldi...ryZ11717QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
Also, there are a few other differences between 620 and 120 roll film besides the width. If you look closely, the 120 is slightly taller than the 620 reel. Film spacing can be a slight issue as well, but usually not enough to matter.
I've tried to modify (with a Dremel) a couple of 620 cameras to take 120 - with varying success, mostly of the no-freaking-way variety. Better to reload 120 onto 620 spools, I think.
Best Regards,
Bill Mattocks