this is quite simple, just tell the OX user account which IMAP Server to use. This can be done with
Just relogin then and the user should be able to use the local or remove IMAP/SMTP Server. Note that the users password of the OX account and the IMAP/SMTP account must be equal.
/opt/open-xchange/sbin/changeuser -c 1 -u username --imapserver yourserver.com --smtpserver yourserver.com --imaplogin username_at_imapserver
More information: http://software.open-xchange.com/OX6/doc/
Hope that helps