Documentation of hook_field_settings_alter

I was looking for using hook_field_settings_alter() to implement more data into a cck field by an extra module.

For this reason I tried to use something like the following to implement some new database entry to store the additional data.

<?php 
hook_field_settings_alter(&$settings, $op, $field) {
  if (!$condition) {return}
  switch ($op) {
    case 'database column':
      return array ( ... )
  }
}
?> 

But this did not work. Both hooks hook_field_settings_alter() and hook_widget_settings_alter() lack sufficient documentation. Is it simply not possible to add more db columns this way?