Create an unmodeled Node and two unmodeled Links that are just like what you expect will exist if the drop happens. Put the temporary Node and Links in the “Tool” Layer so that they are in front of everything and so that those changes aren’t recorded in the UndoManager.
Unrelated example of adding unmodeled Nodes and Links to a Diagram: Unbound Parts
Unrelated example of adding unmodeled Links to a Diagram: A Tree with Sequencing Links between Sibling Nodes
Unrelated example of adding unmodeled Parts to a Diagram: Benchmark