Unfortunately, RadDragAndDropManager does not offer such option. However, you can look at this custom implementation of AutoScroll - http://blogs.telerik.com/blogs/posts/10-11-25/how_to_implement_autoscroll_while_dragging_in_radtreeview.aspx, that allows further customization to fit your needs.
Let us know if you have any further questions.
All the best,
the Telerik team
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