Redmine v2.4.4 Email Watcher-Plugin (Non-Users)

Hello

I'm looking for a plugin to extend the watcher list by emails of non-users. I found this old plugin which does the magic but it is not compatible to my Redmine v2.4.4: https://github.com/edavis10/redmine_email_watchers

Does anyone know of a compatible plugin for Redmine v2.4.4 or knows how to extend Redmine?