The requested feature is not offered by RadEditor for MOSS. If you would like you can render RadEditor as a textbox as shown in the following live demo: Displaying RadEditor as a TextBox
To set the ToolbarMode property to ShowOnFocus, you need to modify the respective ConfigFile.xml which is located in the /Program Files/Common Files/Microsoft Shared/web server extensions/wpresources/RadEditorSharePoint/5.x.x.0__1f131a624888eeed/Resources/ folder.
In addition review the following online help articles:
Set Properties Via Config File
Add/Remove tools from the toolbar
the Telerik team