At present RadEditor does not offer the ability to edit its dropdowns, but you can add RadComboBox control to the RadEditor toolbar and customize it. For a reference, please take a look at the following online resources:
If this does not help, please provide more information about which build in dropdown you have to customize and what you want to achieve.
the Telerik team
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to their blog feed