Preferences ==> Adding books ==> Automatic Adding (tab)
Set calibre to automatically add books from your "My Kindle Content" folder.
Then download all your books into the Kindle for PC application.
For the future, go to
http://amazon.com/manageyourkindle and make sure your default device is the Kindle for PC application. (It should be, if it is your only registered device.)
calibre will automatically add all ebooks from that folder, and as you purchase new books, they automatically get queued to download in Kindle for PC, at which point calibre then adds them too.
EDIT: Obviously, you must open K4PC to download books, and open calibre to import them.
But other than actually launching the applications, it is entirely non-interactive.
Auto-added books get deleted from the auto-add folder, so calibre won't add them twice.
This shouldn't be a problem, since the only reason to use Kindle for PC is to download books into calibre -- right?