The short answer to your question is, "Yes, you can add custom fonts to a certificate." I've done it - just need to remember how I did it!
A Moodle Certificate is a PDF file created using TCPDF (I believe). There are several issues to consider:
a) What are the default fonts used by TCPDF?
b) What additional fonts are installed in Moodle?
c) What fonts does the end user have installed on their computer or printer? This is important, because it is possible to specify a font in a PDF file that cannot be displayed or printed by the end user if the user does not have access to the font.
The situation I had was a requirement to use an uncommon font in a certificate. Since I had to allow for cases in which the end user did not have the font installed on their computer, I used Moodle's embedded fonts code as my base. As I recall, I did the following:
- Installed the font file in Moodle at .../lib/tcpdf/fonts
- Made a copy of the certificate.php file from folder .../mod/certificate/type/letter_embedded. If you want metric measurements use .../mod/certificate/type/A4_embedded
- Followed instructions from TCPDF on fonts and embedding fonts at http://www.tcpdf.org/fonts.php
- Made all necessary changes to the certificate.php file to get the certificate as I wanted it.
- Created my own certificate type folder in .../mod/certificate/type and added my customized certificate.php file to that folder.
You may have to dig around in the TCPDF site and Moodle forums for additional details. But what I ended up with worked.