I've been using Glyphs on a Mac to create web-icon fonts for the site I make, but it also seems to be able to edit fonts (most of the time). I don't really know a ton about font-making, but it seems that with most of the fonts you'd edit, the kerning and all that is already defined.

If it's a matter of adding a couple slashes to 7's/Z's or to 0's, that should be reasonably straight-forward to do. In Glyphs, it would be a matter of adding a new shape for the slash wherever you want it. Then File -> Export and you're on your way.

(And obviously you might run into license violations if you're changing fonts you're not allowed to. This post isn't meant to address the legal side of it, just a guy at his desk fixing fonts for his own use.)

(Edit: typo.)