This feature is not supported by RadControls for ASP.NET AJAX.
If you purchase, however, a license for RadControls for ASP.NET, you will obtain the source code of the controls and you can customize it to achieve your requirement. You can find the pricing of Telerik products in the Telerik's online store
All the best,
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