Hi,
First please note that not all email clients support web fonts. So even if you add a google font correctly to your HikaShop emails, many of your customers won't see the font. So you need to be careful and do tests on different clients to check the results so that it displays fine even when the web font is not supported.
You can read more about adding web fonts like google fonts to emails here:
www.emailonacid.com/blog/article/email-d...-fonts-google-fonts/
With the methods explained there, you can then edit your HikaShop emails via the System>Emails menu (only if you have HikaShop Business). There, use the edit button of the template setting and you'll get a popup with the template. There, you can add your custom CSS to load your web font and use it for the text of the emails using the template.