Take a look at the client API of the two specified controls:
Inspecting more closely the methods and properties provided for the client objects of the RadComboBox and RadMenu you should be able to achieve the functionality you need to implement.
I hope this will help.
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