There are mainly two types of passwords in Gluu Server: the ones that are encoded by its own code (like OIDC clients' secrets, or credentials to connect to LDAP servers in different configuration pieces), and the ones that are generated by LDAP server itself (local user passwords).
Normally, both of the password types are salted, each with its own salt. Importing salt used by Gluu Server's component is more or less straightforward process, but for OpenDJ it may not be such - especially with such version gap as in your case.
Where do your user come from? If you create them manually, there should be too much of them, and may be it would be possible to just re-set all passwords manually? Or prepare a person auth script that would ask them to set a new password at first login.