Hello,
Unfortunately, this is not supported at this stage. Could you please give us more information about your scenario? If you want to use the RadComboBox with WCF service, please refer to the Valeri Hristov's blog post - http://blogs.telerik.com/valerihristov/posts/08-12-04/auto-complete_combobox_with_wcf_service_and_radcontrols_for_silverlight.aspx. The example is for Silverlight, but you could get the main idea.
I hope this helps.
Regards,
George
the Telerik team
Registration for Q1 2011 What’s New Webinar Week is now open. Mark your calendar for the week starting March 21st and
book your seat for a walk through all the exciting stuff we ship with the new release!