You will not be able to select any item if no items have been previously added to the combobox. You can, however, set some text to the input field when the page is first rendered. For example, you can use the pageLoad event and set some default text to the input part of the combobox, like:
var comboBox = $find("<%=RadComboBox1.ClientID"%>);
Additionally, you can set the default text only if the items have not been loaded yet:
I hope that this is acceptable for you.
the Telerik team