I've just started lerning Doctrine and Symfony
and now I want to add a method to the nested
set-class of Doctrine but I really don't know
how to implement it.

So... here's the method a want to add:
(As you can see it's just the "getDescendants"-method
with an option to sort the result by a specified column)

public function getDescendantsSorted($depth = null, $includeNode =
false, $orderBy = '', $orderType = 'ASC')
{
  $baseAlias = $this->_tree->getBaseAlias();
  $q = $this->_tree->getBaseQuery();
  $params = array($this->record->get('lft'), $this->record->get
('rgt'));

  if ($includeNode) {
    $q->addWhere("$baseAlias.lft >= ? AND $baseAlias.rgt <= ?",
$params)->addOrderBy("$baseAlias.lft asc");
  } else {
    $q->addWhere("$baseAlias.lft > ? AND $baseAlias.rgt < ?", $params)-

  if ($depth !== null) {
    $q->addWhere("$baseAlias.level <= ?", $this->record['level'] +
$depth);
  }

  if ($orderBy != "")
    $q->orderBy("$baseAlias.$orderBy $orderType");

  $q = $this->_tree->returnQueryWithRootId($q, $this->getRootValue());

  $result = $q->execute();

  if (count($result) <= 0) {
    return false;
  }

  return $result;

Thanks in advance!

Add a behavior MyNestedSetBehavior derived from
Doctrine_Template_NestedSet and override the setup method and relpace
$this->_table->setOption('treeImpl', 'NestedSet'); with
$this->_table->setOption('treeImpl', 'MyNestedSet');

After, you replace actAs : [NestedSet] with actAs :
[MyNestedSetBehavior]

I have not tried it but it should work.

Note that if you just want to add an orderBy clause, you should be able
to do it by calling the setBaseQuery($q) method of the Tree just before
you call the getDescendants methos. (Where $q contains the orderBy
clause)