We recommend you use the Radiant Logic Virtual Directory Server, and then you can use the VDS Cache Refresh setting in oxTrust. Radiant Logic VDS is a commercial product, but it is a very productive tool for mapping and joining the data as needed.
Alternately, you need to write some kind of sync script to get the data into the Gluu OpenDJ server. You can use either the Gluu SCIM 2.0 or LDAP interfaces for this purpose. For example, consider using a python script, with the Set operations (Set A - Set B) to detect changes.