Hello,
Sorry, but there is no such option that could allow to add google fonts by adding a filter. It should be added globally in XStore Core plugin files.
For this time you have a few options:
1/ you may use Custom fonts as was answered previously
2/ Follow the instructions here -> https://ostraining.com/blog/wordpress/adding-google-fonts/#:~:text=Enqueue%20Google%20Fonts%20in%20Your%20Site (method #2 of using Google fonts with PHP enqueue functions)
Also, you may prebuild a list with fonts and send it to us to include globally. But keep in mind we are able to add a few fonts but not 1000s of them. (There is no need for all customers to load many more fonts in the customizer)
Regards