The right way to do this is to figure out how to get the WD to respond to filesystem events and have MC run auto import by itself. I don't have any experience with that, so I can't help you with that just yet.
For running auto import manually, you can of course use the fairly quick path to open it up: Tools > Import... . Then select run auto import now. There's also a core command for running auto import. Because there's a core command you can map it to a hot key if you want to. Mapping keys takes some extra effort, but it's not so bad.
Hopefully someone else will have a good idea of how to make it work automatically. If not, we can fall back to mapping a key, or something else.
Brian.