Unfortunately, currently our RadListBox control does not support the scrolling that you are trying to achieve. You can check our on-line demos
for more details on the appearance of the RadListBox that you can get. I will however forward your request to our development team so they can consider it for future implementation.
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