Modifying 'top' objectClass

Can I modify the 'top' objectClass in openLDAP to incorporate some activedirectory attributes. Existing schemas in '/etc/ldap/slapd.d/cn=config/cn=schema' can be modified using ldapmodify. But i cannot find 'top' objectClass in any of these. To my knowledge, 'top' is part of system schema but can it be modified?