doveadm sync different local vs remote account/mailbox name

I'm, trying to sync/backup mailboxes from a current dovecot instance to a new server. But the old server has real system accounts. The new server has virtual users. And the account/user-name is different.

Example: Joe Sixpack is joes (on the old system) but is joes@sixpack.org (on the new system.)

I'm using something like this to attempt the sync doveadm -o mail_fsync=never backup -R -u joes@sixpack.org imapc:

This works from the new system, as it recognizes the user with a full @domain.com username - but the old system doesn't know who joes@sixpack.org is - and just wants joes.

I don't see a way to specify the remote account username independently from the local one. What's the best way to handle this?

TIA -Greg