Drupal 7 views 3 add custom filter handler for an existing field
To add a Drupal 7 views custom filter handler for an existing field, you should implement the following hooks.

//in yourmodule.module
/**
 * Implements hook_views_api().
 */
function yourmodule_views_api() {
  return array(
    'api' =&gt; 3,
  );
}

// in yourmodule.views.inc
/**
 * Implements hook_field_views_data_alter().
 */    
function yourmodule_field_views_data_alter(&amp;$result, $field, $module) {
  if($field['field_name'] == 'field_your_existing_field'){
    $name = $field['field_name'];
    foreach ($result as $table_name =&gt; $table_data) {
      if (isset($table_data[$field['field_name']]['field'])) {
        $result[$table_name][$field['field_name'] . '_value']['filter']['handler'] = 'your_custom_handler_in_separate_inc_file';
      }
    }
  }
}

//your custom handler class in separate file
class your_custom_handler_in_separate_inc_file extends existing_handler_class{
// your code goes here
}


//Don't forget to add your_custom_handler_in_separate_inc_file.inc file in your module's .info file like.

files[] = your_custom_handler_in_separate_inc_file.inc