I agree with the answer from the previous post:
https://community.marketo.com/MarketoDiscussionDetail?id=90650000000Pr1oAACIt's not a good idea and you can't do it via Marketo's email editor.
You could specify font via HTML (like below) but that likely won't work if the recipient's mail client doesn't support it. In other words, it's strongly recommended that you don't do it!
<span style="font-family:'Montserrat';">Text</span>