I was recently futzing around with two copies of my EF libraries, trying to get them in sync, when I tried to drag a couple files from one to the other, and then saw that I couldn’t. First I dragged it from one window into the source pane of the other, hoping to get it imported into the Library at root level, then onto one of the specific folders of the library. I feel like this would be a nice feature, since one advantage it would have over import from the finder is—theoretically—copying tags and notes as well.
Presumably you’ve thought about this; one question would be what to do with tags that aren’t in the destination library. I guess it would be nice if they were created (with icon, even!) there, but that might be undesirable to some.