I am afraid that I cannot completely understand your task.
RadComboBox does not support two-way binding. In other words, if you type some text that does not match any of the items, this text will not be added to the datasource. You will have to handle this case yourself. You can get the text of the input field using the Text
property of the RadComboBox object and save it in your database.
As for the null values - you could specify empty values that have neither text nor value assigned.
Hope this helps.
the Telerik team