I can suggest you to use the FooterTemplate of the RadListBox to decelerate a Button, which click handler would initialize a request to the DataBase for additional RadListBoxItems. I have prepared a sample project, demonstrating how to implement the above suggestion.
Note : dll files are removed from the attached sample.
the Telerik team
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to their blog feed