fetching user details from a ldap entry which consists of an objectClass other than posixAccount

I am using libnss-ldap in Debian Lenny. My LDAP user account entry does not contain the objectClass posixAccount. Instead it belongs to another objectClass sCSPerson. Adding both the object classes to the user account entry is not an option. I would like to know the configuration parameter to be used in libnss-ldap.conf so that the user details are fetched from the entries whose objectClass is sCSPerson rather than posixAccount. I have tried the below configuration parameter but it does not work:

nss_base_passwd     ou=People,dc=synovel,dc=com?sub?objectClass=sCSPerson

Are there any other means of achieving the same?