Using dsync for a single mailbox?

[Wim Lewis]

Is it possble to use `doveadm sync` to migrate a single mailbox? I'm 
migrating accounts from another IMAP server,and most of the mailboxes 
are in mbox or maildir format, but a few are in MBX. As far as I can tell
the only recommended way to "convert" an MBX mailbox is to sync it via
imap. But dsync fails in inscrutable ways, and the documentation is kind
of scant on how to control what it does.

Even just an example command line showing how to copy remote mailbox FOO 
into dovecot mailbox BAR would be extremely helpful.