Well you might not think this is obvious but what you could have done is simply use an IMAP client like Thunderbird and “move” the messages from one account to the other. Well given the original host was using IMAP too.
With dreamhost, each user gets their own home directory. This goes for ssh, ftp, shell and mail-only users. And thus each user has their own Maildir.
AFAIK you would have to untar the messages to the Maildir of a shell user, then login to IMAP as that user and that of the destination user, and “move” the messages between accounts.
Also see E-mail Import
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