An example of how to create a new Drush command which wraps an existing command.

sql_sync_wrapper.drush.inc.php

<?php
/**
 * @file
 * An example of how to create a new Drush command which wraps an existing command, allowing some customisation.
 *
 * Note any file containing this snippet should be suffixed '.drush.inc', not '.drush.inc.php'.
 */

/* Implements COMMANDFILE_drush_command(). */
function sql_sync_wrapper_drush_command() {
  $items = array();

  /* Get the set of commands in which the one we wish to call resides. The function
   * is named COMMANDFILE_drush_command() so using something like:
   *
   *  - grep -RB500 "\['sql-sync'\] = array" /usr/share/php/drush/commands | grep function | tail -1
   *
   * will help dig this out (if it's a core Drush command).
   *
   * @see http://drush.ws/docs/commands.html
   */
  $sql_commands = sql_drush_command();

  if (isset($sql_commands['sql-sync'])) {
    // Set up our new command based on the existing one...
    $items['sql-sync-wrapped'] = $sql_commands['sql-sync'];
    // ...and overwrite any elements necessary.
    $items['sql-sync-wrapped']['description'] = 'Do something then copy and import source database to target database. Transfers via rsync.';
  }

  return $items;
}

/* Initialise function for the new 'sql-sync-wrapped' command. */
function drush_sql_sync_wrapped_init($source = NULL, $destination = NULL) {
  // We can implement drush_COMMANDFILE_init() to perform any other tasks or checks.
}

/* Callback function for the new 'sql-sync-wrapped' command. */
function drush_sql_sync_wrapped($source = NULL, $destination = NULL) {
  // Do whatever we need to...
  // ...

  // Then call the command we're wrapping. We pass the arguments directly, whilst
  // the command options used will be the same as those passed on the command line.
  drush_invoke('sql-sync', array($source, $destination));
}