Google fonts are available in the landing page text editor. If you want to use a web font that isn't listed, it's possible, but you'll need to insert it manually.
You can't add anything to the rich-text editor font dropdown. What you do is insert the appropriate reference to the web font in the <head> section of your landing page template (or individual landing page), then in the template specify that the editable text element should use your web font as the first choice. The rich-text editor will just show "Default" meaning it will just inherit whatever the template is enforcing on that section.
if you have
<div class="mktoText" style="font-family:myWebFontName, Arial, sans-serif;">My Text Element</div>
Then, that div will be using your web font and when you open it up in the rich-text editor it will just say "Default".
I had the same idea and it also works. But that way you can only add one font.
I also tried to override the fonts in the dropdown menu by import a font and use the same name. But doesn't work. Maybe some kind of force is needed.
You actually can add as many as you want. If you're comfortable, you could easily wrap your entire rich-text section in a <div> with whatever font-family you want from within the HTML editor of our rich-text editor. Alternatively, you can use a "String" variable in a guided landing page to allow the user to type in what the font should be for various sections that you could code into your template...