Yes. In order to have this available, however, you must add a “main font” via your Design Settings on your Guideline. Once this is set, the font will be set as your default font within your Newsletter.
We recommend that you consider the fact that the email HTML Standard is very inconsistent and not reliable when it comes to rendering custom fonts. This is an industry problem and not specific to Frontify's Newsletters. To ensure that your fonts show correctly, we recommend using style guides with system fonts. There are a variety of system fonts that are preinstalled and run reliably on every machine.