Just add three more lines:
$data = $xml->createElement('enclosure', $rssdata["nNr"]);
$enclosure = $xml->createAttribute('type'); # creating attribute 1
$enclosure->value = 'image/jpeg'; # setting value
$data->appendChild($enclosure); # adding attribute to element
$url = $xml->createAttribute('url'); # creating attribute 2
$url->value = ''; # setting value
$data->appendChild($url); # adding attribute to element
$item->appendChild($data);
If you have many attributes to add, you might consider creating a function to reduce code duplication.
Your variable names may be confusing. If you rename $enclosure and $data your code might become clearer and easier to read:
$enclosureElement = $xml->createElement('enclosure', $rssdata["nNr"]);
$typeAttribute = $xml->createAttribute('type'); # creating attribute 1
$typeAttribute->value = 'image/jpeg'; # setting value
$enclosureElement->appendChild($typeAttribute); # adding attribute to element
$urlAttribute = $xml->createAttribute('url'); # creating attribute 2
$urlAttribute->value = ''; # setting value
$enclosureElement->appendChild($urlAttribute); # adding attribute to element
$item->appendChild($enclosureElement);