The requested feature is not supported by the built-in dropdowns of RadEditor. If you would like you can add RadCombobox to the toolbar of RadEditor and implement the requested feature. You can see how to add the control to the editor's toolbox in this article: Can I add a combobox to radeditor's tools collection?
and how to insert content in RadEditor: Pasting content from RadComboBox item in RadEditor at cursor position
All the best,
the Telerik team
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