Good Evening,
with a little help from Ralph I have now successfully setup LDAP bind on the phpBB TestVM. The plugin seems to be read-only, so we need to find another way to register user accounts (maybe from CMS).
Next step would be a test database import from newbb to make sure that existing posts can me migrated. Chee already provided a script for converting newbb to phpBB2 so maybe I have to migrate to phpBB2 database format first and then upgrade to phpBB3 (unless a SQL guru want's to spend some time on it).
Best Regards Marcus