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.
the Telerik team
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