Using foreign ldap catalog in Exchange 2003 environment

Hi,
Our hospital is coming to be married with another. We use Exchange 2003 with AD and they use an opensource mailing system with an ldap catalog. I want to give my users ability to consult and use this catalog. I know, there is a way to configure addresses books in Outlook, but it's not easy to change all outlook's configs specially for that feature. Does anyone know a mean to add this catalog directly in Exchange ?
Thanks.