To add new brand xml namespace use:
<?php
    $element->setAttributeNS(
        'http://www.w3.org/2000/xmlns/', // xmlns namespace URI
        'xmlns:mynamespace',
        'example.com/mynamespace'
    );
?>
'http://www.w3.org/2000/xmlns/' URI is important
to be able to add  new namespaces !!!
Later you can use your namespace like:
<?php
    $element->setAttributeNS(
        'example.com/mynamespace',
        'mynamespace:something', 
        'value'
    );
?>