Using dsync for a single mailbox?

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.