I am sorry to say that this functionality is not available in the control yet. For the time being you can monitor, comment and vote on PITS item here
to raise its importance. If the item gets enough votes and approval from the management you can expect it in the near future.
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