Hello,
The easiest solution is to use the embedHTML method adding <br> tags:
<?php
require_once 'classes/CreateDocx.inc';
$docx = new CreateDocx();
$wf = new WordFragment($docx, 'defaultHeader');
$wf->embedHTML('<br><br><br><br>');
$docx->addHeader(array('default' => $wf));
$docx->addText('Document content');
$docx->createDocx('output');
Or you could use addBreak to add breaks in the WordFragment added to the header.
If the number of headers is limited, you could created empty DOCX templates with the headers and import them using the importHeadersAndFooters method. This method removes existing headers.
Regards.