Perhaps in a person authentication script, when the person logs in, read the attribute from LDAP, and write it to the other attribute. So it would be "just in time". There are many sample interception scripts here:
https://github.com/GluuFederation/oxAuth/tree/master/Server/integrations
and you can read the docs
https://gluu.org/docs/gluu-server/4.2/admin-guide/custom-script/