Adding more values to Drupal user status
By default, I don't think there's any way of doing this.  The user status is used by other parts of user.module (as well as x amount of contrib modules), so extending it could cause conflicts there.

What you could do however, is add a custom status field to your users and use that.  You'll get most of the normal benefits of the Drupal user status, but it'll be as extensible as you need.

Combine it with Field Permissions to prevent users from being able to see or edit (or either one of those options) themselves should you need to.


  The Field Permissions module allows site administrators to set
  field-level permissions to edit, view and create fields on any entity.


If you still want a user status that disabled the user account, then you could create a value on your custom status field (let's call it Disabled and then configure a Rule that says:


  Event:
  
  After saving a user account After updating a user account
  
  Condition:
  
  Use data comparison to se if *field_my_user_status* is equal to
  disabed
  
  Action:
  
  Block user account


*NB If you do this, don't forget to create an opposite rule that enables the user account based on your field value :)