In addition to the icon-sets included. If you’re loading a custom icon font to your site, you can also use it for icons, by way of adding class names. Provided that the styles are structured in the standard way most font icon styles like Font Awesome and Fontello use. You can simply add the classes to the Icon (custom) option in the Icon / Image Selector. When you’ve added the classname, click on Set icon button. Icon color option also works for these icons.
Make sure that the icon styles are already loaded on the page. It is preferred to add them to the WP Admin as well , as it makes it possible to preview the icons on the Form Editor.
Advanced users: Loading custom icon fonts.