Create single ftp user w/o domain

I need to use my webserver as a backup server for one user.
What would be the best way to create an ftp user with access to /mnt/backup/username?
If I understand correctly, proftpd reads from sql-database only so I can’t just add a linux user, in siteworx all users are connected to a domain with no access outside /home. (Tried w symlink to /mnt/backup/username but not allowed/doesn’t show.)

And just for clarification, partly OT: is it safe to add users using cli?


Actually by default, the sql-database is checked first, but it will try to use the unix users on the system if they exist and have a password set. So you could manually add a unix user, set his home directory to be the location you want the files to go, and set a unix password for the new user, and you should be able to login via FTP as that user.