How to add additional font?

Your question is out of the scope of our plugin, which does not alter the WP visual editor, but it just uses it for widgets.
If you want additional fonts in your editor you should add custom code (i.e. see this article and this other), or use a plugin (i.e. Ultimate TinyMCE has a commercial addon for that).
If the font you want to use is not a Google web font, you should convert it to a web font format (i.e. using Font Squirrel Webfont Generator) and adjust the custom code.