The only way to implement this functionality would be to download the source code and apply the changes there. You can find the source code of RadControls for Silverlight from your account, Download and Manage your products" -> "RadControls for Silverlight".
However, you should have in mind that tables are one of the most complex document elements and the implementation of this feature would require an in-depth understanding of the whole rich text box control. Furthermore, the source code is not supported and any changes you implement may be incompatible with the following versions of the control.
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This feature has not been implemented yet and at this point is not scheduled so I cannot say when it might be available. Here is the correct link where you can track our progress in the Feedback & Ideas portal.
Let me know if you need additional information.
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