Could you please create a CssEditor.css file in the following directory
\Program Files\Common Files\Microsoft Shared\web server extensions\wpresources\RadEditorSharePoint\126.96.36.199__1f131a624888eeed\RadControls\Editor
and put your custom css classes in it. This file is not
created by default when you install
RadEditor for MOSS. However, if it is present, the editor will automatically
load it. Note that the folder name depends on the editor version (in this case
it is 188.8.131.52). Also do not forget to back up the CssEditor.css file when you
uninstall/upgrade the editor.
The classes defined in the CssEditor.css will be loaded in the Apply Class dropdown as well as the css class dropdown of the table wizard and it will list the classes for TABLE, TR, TD and all elements.
I hope this will help you to achieve your scenario.
the Telerik team