I apology for misreading your question. About SFTP, the users with access to SFTP are the system users too, so you can add as I told above. But you don't have to modify the wp-config.php file.
To restrict the access, you can edit the /etc/ssh/sshd_config file. You can add the following group at the end of the file:
Subsystem sftp internal-sftp
Match Group onlysftp
Then, you have to create the group and asign the previous user (customuser) to this group:
sudo groupadd onlysftp
sudo usermod customuser -g onlysftp
And finally, deny SSH shell access and set its home folder to the wordpress directory:
sudo usermod customuser -s /bin/false
sudo usermod customuser -d /opt/bitnami/apps/wordpress/htdocs
After doing so, you have to restart SSH server:
sudo service ssh restart
I hope it helps.