The ability to change WYSIWYG Editor is provided in Site Preferences.
The textarea needs to be an editor for
1. Complete Description for EventInfo form
2. Pre-form Help & Post-form Help for Custom Data Group
3. Pre-form Help & Post-form Help for Profile
4. Message Templates HTML version
5. CiviMail editor / Send an email
The method for adding editor addWysiwyg is in CRM/Core/Form.php. Check the comments for the usage.