Registration field not to be saved
Yep!
Take a look at:

yourtheme_form_user_register_form_alter


Add a new value to the form there,  and then change the form's saving function. In your new saving function you can do whatever you need to do with custom validation and re-assigning that value.

https://api.drupal.org/api/drupal/modules!system!system.api.php/function/hook_form_alter/7