Adding the custom schema went fine apart from the replication issue:
----------
dn: cn=schema,cn=config
changetype: modify
add: olcAttributeTypes
olcAttributeTypes: ( 1.3.6.1.4.1.4203.666.1.200 NAME
'mailacceptinggeneralid' DESC 'Postfix mail local address alias
attribute' EQUALITY caseIgnoreMatch SUBSTR caseIgnoreSubstringsMatch
SYNTAX 1.3.6.1.4.1.1466.115.121.1.15{1024} ) olcAttributeTypes: (
1.3.6.1.4.1.4203.666.1.201 NAME 'maildrop' DESC 'Postfix mail final
destination attribute' EQUALITY caseIgnoreMatch SUBSTR
caseIgnoreSubstringsMatch SYNTAX 1.3.6.1.4.1.1466.115.121.1.15{1024} ) -
add: olcObjectClasses
olcObjectClasses: ( 1.3.6.1.4.1.4203.666.1.100 NAME 'postfixUser' DESC
'Postfix mail user class' SUP top AUXILIARY MAY ( mailacceptinggeneralid
$ maildrop ))