Hello Ludovic Alcala,
Yes, it is possible to disable before / after dropping by handling the DropQuery event and specifying that if the DropPosition is Before or After the drop should not be possible.
There is a help articles that covers this here:
If you would like to do something specific depending on a pressed key, you can check the Keyboard.Modifiers
and decide what to do depending on the modifiers. A good event to handle there will be either the treeView.DragEnded or RadDragDropManager.DropInfo.
This article details implementing copy-drag:
Hopefully this will help you,
the Telerik team