No need to share folders. Every user account has a Junk folder. Dragging an email into or out of the Junk folder automatically forwards it to the spam or ham account as appropriate.
This has been standard behavior for a long time (5.0.11 I believe). See
Bug 9532 – IMAP/Outlook move to junk doesn't train anti-spam
In Outlook, unfortunately, there's no single button that moves an email into/out of Junk, but dragging works fine. You can also right-click and use "block this sender" but this is somewhat misleading, as the sender isn't blocked although the mail does get put into Junk, which triggers training.