OpenDJ does not support non-salted SHA256. What I would do is to store these passwords in another LDAP attribute, implement a comparison in a Person Authentication Interception Script (where you implement the SHA hash prior to the comparison), store the password in the `userPassword` attribute using the default hash (bcrypt), and remove the legacy password hash.
Writing scripts and helping with custom migrations is outside of community support. But you could reach out to one of our partners, like Centroxy, for help.