Order Export:Custom Variables in Filename
From XTENTO Support Wiki
Want to add a new custom variable to the filename of the exported file, generated by your XSL Template?
You cannot define "filename variables" from within XSL. That must be done from PHP:
See: \app\code\Xtento\OrderExport\Model\Output\AbstractOutput.php
Within the replaceFilenameVariables function you can add custom variables.
Preferably do this using the event which is dispatched there: xtento_orderexport_replace_filename_variables_before