I am not sure what exactly you need to implement, but it might consider using the client API to add items on the client:
Also, if you item is selected like in when you have EnableLoadOnDemand="true" you can use the EmptyMessage
property. For example, EmptyMessage="custom text...".
If these suggestions don't help you with your requirement, please provide more details and we will try to find an appropriate solution.
the Telerik team