If your desired font is not available to add as a Google font, it might be possible to upload it in “ProPhoto > Customizer > Design Group > Fonts > Custom Fonts”. Up to five .woff font files designed for web use can be uploaded here. 
If you have a valid web use license for a TTF or OTF file, or that font’s usage policy allows web use, you should be able to convert this to a WOFF file using any number of online converters. The converters at andrewsun.com and everythingfonts.com seem particularly easy to use. If you already have a webfont kit zip file, inside the zip you should see a .woff file.
contents of an unzipped webfont kit zip file
However you acquire it, a proper .woff file should upload just fine. After it’s been uploaded, you’ll see it available for selection when creating font styles. 
