I want to create an action which sends an email, but it is different from Drupal's built-in email-action. My question is that how should I write one extra action (which is advanced based on Drupal's definition) and add them to this list?
Should I write a module?
Or should I edit the existing files? (which oh! weird)
Or there is already a way to do this?
The examples module has a good explanation of creating a custom action which can be found here
Otherwise you could have a look at using [Rules module][2]
[2]: https://www.drupal.org/project/rules to create your functunality.