How to add Google Fonts in TinyMCE Advanced?
I was looking for the same thing exactly, until I came across this plugin:
Just install it (after you already have TinyMCE Advanced, of course) and checkout its code file (under Plugins -> Edit).
All you have to do is add the font definition (name) to the list of fonts in the designated code (located near the end of the main PHP file of this plugin) and add the list item code to another place in the code (a little bit below the first one).
That's it! Now you have the additional font(s) in the dropdown of the font family selection in the editor.