Could you clarify some aspect of your scenario?
First how do you add the RadComboBox, is it in some server-side event or on initial page load?
In case you make a post-back from the page and you want to update only a particular part of it, you should make an ajax request and thus RadAjaxManager should be used. So please clarify what is the exact scenario that you are trying to implement.
All the best,
the Telerik team