Migrating D5 users to D6 using MigrateDestinationUser
As per http://drupal.org/node/1349758, you need to add a complete function:

$this-&gt;addFieldMapping('pass', 'source_password');
...
public function complete($account, $row) {
  db_update('users')
    -&gt;fields(array('pass' =&gt; $row-&gt;source_password))
    -&gt;condition('uid', $account-&gt;uid)
    -&gt;execute();
}