Hi Mike 
If I understand well, you created a custom user field that is currently empty and wants to fill it in for each existing user based on data you have somewhere.
I’ll do that with a rail script, and I’ll check the content is unique inside the script.
Since it was migrated from phpBB, do you have any reason to believe phpBB users’ IDs wouldn’t be unique?
If you’re Discourse-hosted, I’d ask directly to team@discourse.org as you don’t have access to the rails console; maybe experts will have better solutions.