1) create a normal ftp user in plesk.
2) edit /etc/passwd eg
sudo vi /etc/passwd
3) find the new user
4) change that users entry from
ftp_user:x:10002:1003::/var/www/vhosts/mydomain.com:/bin/false
to
ftp_user:x:10002:1003::/var/www/vhosts/mydomain.com:/bin/bash
5) restart ssh eg
service ssh restart
This will allow that ftp user to access ssh inheriting the same permissions as the default subscription user.
Note this type of user cannot have sudo access.