Function corresponding to admin/content/simplenews/users

Unfortunately I need to do a hack this module to create a specific filter in the list of subscribers (admin/content/simplenews/users/list), but it is difficult to find where it is located.

Could someone tell me what the name of the function that allows me to create an additional filter in admin/content/simplenews/users/list (not need to be an exposed filter, I can directly change the SQL)?