Just leaving this here...
I'm sure it will help someone else.
Here's how I added an emergency user to my Plone 6 site.
cd backend
docker compose exec backend bash
source docker-entrypoint.sh
bin/addzopeuser -c etc/relstorage.conf emergencyadmin password
Note:
This assumes your backend is managed using docker compose.
If you use the make start-backend command, this is very likely.
Running source docker-entrypoint.sh ensures that key environment variables are set, the addzopeuser command will fail if these are not set.