Yes, PowerToys is a free and very handy utility for Plesk servers.
The only other way to edit the subjects would be to manually make changes to the 'psa' database, in the 'notifications' table.
SideNote: I do wish that SW-Soft would use better grammar! They have such a nice interface for the clients to use, then mess up the professional feel by having lousy English grammar which makes some things seem 'child like'