How to add extra option to the existing drush command?
This is done with drush_help_alter rather than drush_command_alter. See the example Drush policy file, reproduced in part below:

function policy_drush_help_alter(&amp;$command) {
  if ($command['command'] == 'updatedb') {
    $command['options']['token'] = 'Per site policy, you must specify a token in the --token option for all commands.';
  }
}