How can I add a new font type to Sugar's tinymce editor? 

In \include\javascript\tiny_mce\themes\advanced\editor_template_src.js

I made some changes to the init function to add Calibri:

t.settings = s = extend({

  [...]

  theme_advanced_fonts : "Calibri=calibri,helvetica,sans-serif; [...] "

  [...]

}, s);

[... and later on ...]

ed.contentCSS.push("http://fonts.googleapis.com/css?family=Calibri:300,400,700");

Obviously, that's not enough.

What's missing ? Or is my whole approach wrong?

Is there any way to do this just by changing \custom\include\tinyMCEDefaultConfig.php?