Despite the fact that the name suggests different, "Font"Awesome is not really a font. It's an icon set, that happens to be stored in a font file, because that's the most convenient way of delivering them, but calling it a font is kinda like putting a sandwich in a pizza box and start calling it a pizza. As such, there probably are ways to use it as a font, but it's not intended, nor recommended. You can use the editor manager to deliver the font to the frontend, but for one it's already delivered through XF natively, and for second, it won't become usable from the font dropdown. You're probably better off just creating a custom BB code that takes the icon name as parameter and replaces it with the FontAwesome html code.