Please review the following help articles.
1. Add/Remove tools from the toolbar.
2. Add Custom Buttons.
What you should know is that if you are using the classic ASP.NET version of RadEditor for MOSS the configuration files folder of the control is /Program Files/Common Files/Microsoft Shared/web server extensions/wpresources/RadEditorSharePoint/188.8.131.52__1f131a624888eeed/RadControls/Editor/
I hope this helps.
the Telerik team