Yes it is possible to place a RadComboBox inside the RadXmlHttpPanel. When using any RadControl inside the Panel you need to set the RegisterWithScriptManager property to false. Also set the EnableClientScriptEvaluation
property to true, so that the client scripts loaded from the RadComboBox are evaluated and you can use its client-side methods and properties.
Do not hesitate to contact us if you have any other questions.
the Telerik team