Unfortunately the described scenario is not possible in the current version of the control.
By design the scroll wheel is designed to be used to change the value of the control only when the mouse is directly above the control. Making it to be used no matter if it is above the control could cause serious issues. For example when the RadNumericUpDown is placed in a ScrollViewer in a Window, if the control steels the scroll wheel to change its value the result will lead to no scrolling of the Window's ScrollViewer.
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