I think I've asked for that years ago and I work around the issue everyday.
 

It's possible to drop marked items under a selected item (which is logical), but it's not possible to paste under a selected item (unless one creates a bogus item under that item, select the bogus item to be able to paste items "as children" of the previously selected item, etc.).

Anyway, it would be great.

I'm just not sure how to implement that without creating a monster "paste dialog".