Unfortunately currently RadEditor for SharePoint 2007/2010 does not support the required functionality.
What you can do though is to create your own custom version of RadEditor for SharePoint 2010, which supports custom loading of configuration files. We provide the source code of our SharePoint controls on demand, so if you decide to implement the functionality your self, ask for the source in this thread and I will send it to you.
I can give you a few pointers of where in the code the loading of the configuration files occurs, but what you should know is that generally your custom version of the control will not be supported.
the Telerik team
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