You have to edit the javascript file Core.UI.RichTextEditor.js (and you need to edit that file after an upgrade as well). Add the following line after the the „fontSize_sizes“ line:

font_names: "Arial/Arial, Helvetica, sans-serif;Arial Narrow/Arial Narrow, Helvetica, sans-serif; Comic Sans MS/Comic Sans MS, cursive;Courier New/Courier New, Courier, monospace;Georgia/Georgia, serif;Lucida Sans Unicode/Lucida Sans Unicode, Lucida Grande, sans-serif;Tahoma/Tahoma, Geneva, sans-serif;Times New Roman/Times New Roman, Times, serif;Trebuchet MS/Trebuchet MS, Helvetica, sans-serif;Verdana/Verdana, Geneva, sans-serif; Ubuntu/Ubuntu, Geneva, sans-serif",

There you can add fonts.

But the (mail-)clients need the fonts as well to be displayed correctly.