Hello,
There's no option to acomplish what you need, so you should change the library code. You should change the method mergePdf in the MultiMerge class using the SetFont method from TCPDF to embed a new font. This is the signature of the method available in lib/fpdf-tpl/fpdf-tpl.php:
public function SetFont($family, $style = '', $size = 0)
You can read more information about this method on:
http://www.tcpdf.org/fonts.php
Regards.