Expose additional properties for DataFormRadComboBox editor. Properties like DisplayMemberPath, SearchTextPath, etc. 

Currently you can achieve this using the Editor Style 

            <telerik:DataFormRadComboBoxEditor EditorStyle="{StaticResource EditorStyle}"/>
<Style x:Key="EditorStyle" TargetType="telerik:RadComboBox">
                <Setter Property="DisplayMemberPath" Value=""/>
</Style>

or create an editor that inherits from DataFomRadComboBoxEditor and define the DisplayMemberPath property.