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