I was struggling to export Customer ID (entity_id) for some time. I thought it should be straight forward. But it wasn't, so after googling for a while I found this solution.
solution 1:
Override file: app\code\core\Mage\Customer\Model\Convert\Parser\Customer.php
and update function unparse();
previous:
$systemFields = array();
foreach ($this->getFields() as $code=>$node)
{
if ($node->is('system'))
{
$systemFields[] = $code;
}
}