OK, that helps.
Sadly, this won't be trivial.
You'd have to add a Tree Drop event. That event gives coordinates. You can use the coordinates to find out where the mouse is, by using Point To Row Column method. The event allows you to accept or not accept the drop.
Sadly (its all very sad), if you use any drag\drop event, you loose all the build in functionality and you'll have to DIY.
If you want feedback while dragging, you can add a Tree Drag Over event. That event also gives coordinates...