Within Marketing Cloud's email studio, is there a way to add additional font stacks and/or customise the existing font stacks that are applied when you set the font for a block of text or a tag/class?
There are a few ways to use custom fonts in your emails. Here is a great article that details the ways you can achieve this and the pitfalls to watch out for: https://www.campaignmonitor.com/resources/guides/web-fonts-in-email/
This part of the article shows how to import and reference custom fonts.
Make sure you do font compatibility checks for difference devices and browsers.