You can use google fonts in forms already, have a look here:
When it comes to emails, you will need to add it to the html, like so:
e.g. <p style="font-family:Arial">Text</p>
Though a lot of peoples' email clients are set to show emails in a default font. If you really want something to appear in a certain font you may want to consider using an image.
Thanks for your reply.
However for fonts that are not present by default, is there a way to add them into the design studio?
Unfortunately out of the box there is no way to add them.
Perhaps something worthy of an "idea"? :)
Well someone did the raise the idea back in 2012 from what I saw. So now its for Marketo to transform the idea into an actual feature. :)