If you wish to add a font in the list of available fonts inside the text editor in Sitefinity, you will have to do it in the .xml file that drives the editor. Inside the xml, you’ll be able to customise more than just the font list. Take a look at the provided .xml file and it will make sense.
1. Create the xml file to drive the editor: RadEditorXml.xml
2. Add the tool names in the <tools> section
3. Add the font names in the <fontNames> section
4. Add the css in the <cssFiles> section.
(I’ll add an example RadEditor.xml file here as soon as I remember my ftp password 
)
5. Inside Sitefinity, find the Appearence section inside the Advanced Settings.
6. Point to the xml in the StandardEditorConfiguration field.
That’s it. Any font names that you have specified in the <fontNames> section will be available for you to select from the dropdown list on the RadEditor.
There’s great documentation for the RadEditor here.
