This is sort of messy, but it is possible.
As ou already exists in the LDAP schema, you're going to have to do some surgery to get it to work.
1. add ou into the gluuperson objectclass in `101-ox.ldif`
2. Manually create the ldif under `ou=attributes,o=(org-inum),o=gluu` You can copy an single entry in `/opt/opendj/ldif/` give it a new inum, and use `ldapmodify` to load it. Something like `/opt/opendj/bin/ldapmodify -h localhost -p 1636 -Z -X -D "cn=directory manager" -j ~/.pw -a -f "ou.ldif"` where you write the DM password (same as admin password) in `~/.pw` and remove it when you're done.
Keep that ldif handy, and remember to make a similar `101-ox.ldif` every time you update.
Alternately you can make another person objectclass...