Apparently I was mistaken / misguided in my first response - so I'll make an attempt at vindication here ;-).
Building on dlbewley's suggestion, you could also use imapsync to move or copy mail from the users spam directories into the spam mailbox directly. This doesn't solve the password problem entirely, but it does give you a few more password and auth options, which I don't believe you could (easily) have with zmtrainsa directly.
On a related note, I remember seeing a bug lately regarding admin access to user accounts over IMAP. I don't recall if the bug/rfe was for preventing or allowing that, but I'll post back if I can find it in Bugzilla.
--Will
*Update* I found the Bug / Thread I was thinking of
http://www.zimbra.com/forums/administrators/4268-4-0-rc1-imapsync-admin.html#post23931