Hi sebaZ,
There is currently no built in way of doing this, but its not too hard to script it. You can use show grants (SHOW GRANTS · SingleStore Documentation) that will give you a GRANT statement you can run to recreate the user with the proper permissions. You would need to be careful to create roles/groups as well if your cluster uses those.
Users/Grants are server level objects today (not stored within a particular database), so that is why they don’t get store d in backups which happen at the database level.
-Adam