Ok I found a way by myself. I was looking for a way to inject some params while my Zend_Form was initialised. It seems the only way for this is to pass the params to the constructor - which is executed before the init method.
class Form_Payment extends Zend_Form
{
private $_someval;
public function __construct(array $params = array())
{
$this->_someval = $params['someval'];
parent::__construct();
}
public function init()
{
$this->setAction('payment/save');
//....
$this->addElement('multiCheckbox', 'store_id',
array('label' => 'Someval:',
'required' => true,
'multiOptions' => $this->_someval // passed params now available
))
}
}