You could implement the required logic in the events of theDragDropBehavior of RadScheduleView, for example in CanDrop or Drop methods. For more information about customizing the ScheduleViewDragDropBehavior, please refer to the following help topic:
Also in order to check whether the Shift key is pressed, I would suggest that you use the System.Windows.Input.Keyboard class, here is a quick example:
CustomScheduleViewBehavior : ScheduleViewDragDropBehavior
(Keyboard.Modifiers == ModifierKeys.Shift)
//implement any additional logic
I hope this helps.