This functionality is not supported out of the box from the list box. An approach which you could try is to extract and edit the default template of the control. You can take a look at the Editing Control Templates help article in our documentation. In the extracted template you can modify the Grid in the ControlTemplate and add RadComboBox at the end. Then you can subscribe to the SelectionChanged event of the RadComboBox and depend on the selected item add new list box item.
For your convenience, we have created sample project which demonstrates this approach.
Regards,
Dinko
Progress Telerik
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