You can easily make the 12 round 82 magazine into a 13 round magazine. Take your mag apart, remove the follower from the spring, and use a hacksaw to remove the thickness of a 9x18 cartridge from the bottom of the 2 inch long follower. You might even be able to remove enough to allow the mag to hold 14 9x18 rounds. However, removing material from the follower could cause the mag spring to over-compress, shortening the life of the spring.
Before anyone balks at this suggestion, from what I've been told by friends who shoot Glocks, Glock did this with some of their models as a marketing tactic to increase sales be increasing capacity. This was done so they could better compete with dimensionally similar guns be offering higher capacities. It worked too. The draw back is that one must replace the mag springs periodically. Glock recommends replacing their mag springs every 2000 rounds or every 2 years. Again, I don't own a Glock (and won't ever own one). This is just what I've been told.
I did have to remove a small amount, a few millimeters, from one of my mags though. It wouldn't hold the twelfth 9x18 round. The follower in that mag was a touch longer than the followers in my other 82 mags.