can I add a watcher automatically to all issues in selected project?

[Hilary Boyce]

You can create a role  - we have 'project-watcher', then assign users and or groups to that role in the project admin and add the role to the project notification scheme so that they are notified of the same things as watchers.

[Vladimir Yakimov _Botron_]

If the user doing the update is the same as the user you would like to be added as watcher for the issues you can do bulk update and and select "Watch Issues" option.

https://confluence.atlassian.com/jiracoreserver072/editing-multiple-issues-at-the-same-time-829092790.html

[Vladimir Yakimov _Botron_]

Check also this discussion and the add-on suggested

https://answers.atlassian.com/questions/151799