I found some code in another post that solved my problem.
Thanks for your support GreyHead.
<?php
$radio = JRequest::getString('retourrit', '', 'post');
switch ($radio) {
case 'ja':
JRequest::setVar('retour_ophaal_adres', JRequest::getString('bestemming_adres', '', 'post'));
JRequest::setVar('retour_ophaal_huisnummer', JRequest::getString('bestemming_huisnummer', '', 'post'));
JRequest::setVar('retour_ophaal_plaats', JRequest::getString('bestemming_plaats', '', 'post'));
JRequest::setVar('retour_bestemming_adres', JRequest::getString('ophaal_adres', '', 'post'));
JRequest::setVar('retour_bestemming_huisnummer', JRequest::getString('ophaal_huisnummer', '', 'post'));
JRequest::setVar('retour_bestemming_plaats', JRequest::getString('ophaal_plaats', '', 'post'));
break;
case 'nee':
JRequest::setVar('datum_retour', 'n.v.t.');
JRequest::setVar('vertrektijd_uur_retour', 'n.v.t.');
JRequest::setVar('vertrektijd_min_retour', '');
JRequest::setVar('retour_ophaal_adres', 'n.v.t.');
JRequest::setVar('retour_ophaal_huisnummer', '');
JRequest::setVar('retour_ophaal_plaats', '');
JRequest::setVar('retour_bestemming_adres', 'n.v.t.');
JRequest::setVar('retour_bestemming_huisnummer', '');
JRequest::setVar('retour_bestemming_plaats', '');
break;
}
?>