Well with POP your only method would be to forward the spam emails as attachments to the spam.#@domain.com.
You can rename this account if needed:
zmprov renameAccount spam.####@domain.com
spam@domain.com
zmprov ma zimbraSpamIsSpamAccount:
spam@domain.com
Spam/Ham might be to common (and get abused by the wrong people) so we append a random string to the account name - might use like
bad@domain.com or
isham@domain.com etc.
If you could use IMAP this was completed for 5.0.11:
Bug 9532 - IMAP/Outlook move to junk should train anti-spam