Adding new glyphs
In order to add new glyphs you need a font editing application. Glyphs for OS X is by far the cheapest solution.
In this repo, there is a files called genicons.glyphs.
You can compose new vector icons in Adobe Illustrator, create a new cell in Glyphs by clicking the little + in the lower left-hand corner of the app.
Double-click on the newly inserted (but blank) glyphs and copy/paste your vector from AI into Glyphs app. Scale the icon to fit the canvas.
Important: make sure the unicode code falls within the private use unicode range (U+E000..U+F8FF). Also, be sure to group icons by category.
Once you're done adding your new icons to genicons, export the font as an .otf file, an save it somewhere.
Go to Font Squirrel's @font-face generator, which will allow you to convert otf or ttf files to woff, eot, and svg files -- which are all needed for x-browser compatibility.
One can see all current icons here. Each time a new character is added, you must recompile the github page.
On the gh-pages branch of this repo, there is a [icon.yml](https://github.com/geni/genicons/blob/gh-pages/_jekyll_site/icons.yml) file. Following the conventions in that file, add your icons character code plus a description.
Next, recompile the Github pages site. You can find out how to do that here.