With the latest version of the control it is available out-of-the-box. However, I might be missing something and this is why I wanted to ask you to elaborate on your scenario a bit more.
I also attached a sample where the built-in RadPanelBar ScrollViewer is displayed and you can scroll up/down using the MouseWheel. Please have a look at the solution and let me know if this is what you had in mind.
the Telerik team
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