add the following line to ckeditor/config.js

config.contentsCss = 'fonts.css';
//the next line add the new font to the combobox in CKEditor
config.font_names = 'fontnametodisplay/yourfontname;' + config.font_names;

where fonts.css has the [user=font]font[/user]-face attribute:

[user=font]font[/user]-face {
font-family: "yourfontname";
src: url ( ../fonts/font.eot ); /* IE */
src: local("realfontname"), url ("../fonts/font.TTF") format("truetype"); /*non-IE*/
}