I’m pretty sure that this will work:
- freeze the live formerly phpbb Discourse,
- backup
- restore that backup to a development machine
- import the ancient data on dev machine
- backup
- restore to prodcution
- profit
The only question is whether there are any clashes between import ID on the phpBB data and the ancient data. This should be pretty easy to fix in the importer (e.g., add “X” before all of the import IDs).