hook_field_schema_alter() was added in Drupal 7

A new hook_field_schema_alter() was added in Drupal 7. See the example implementation:

/**
 * Allow modules to alter the schema for a field.
 *
 * @param array $schema
 *   The schema definition as returned by hook_field_schema().
 * @param array $field
 *   The field definition.
 *
 * @see field_retrieve_schema()
 */
function hook_field_schema_alter(&$schema, $field) {
  if ($field['type'] == 'image') {
    // Alter the length of a field.
    $schema['columns']['alt']['length'] = 2048;
    // Add an additional column of data.
    $schema['columns']['additional_column'] = array(
      'description' => "Additional column added to image field table.",
      'type' => 'varchar',
      'length' => 128,
      'not null' => FALSE,
    );
    // Add an additional index.
    $schema['indexes']['fid_additional_column'] = array('fid', 'additional_column');
  }
}