The Brand Kit features 100+ free fonts to use, as well as the option to set a font pair to apply to all your designs (click the Edit brand button in your Brand Kit to do this!).
However, if you've purchased a custom font and have the license to use it, you can upload it to the Brand Kit and use it for all your designs.
Importing a font file
From your Brand Kit, click a design to edit (e.g. a social post).
Click the text in the design. In the left menu, click the font used.
Under My Fonts, click the Add button.
Select the font file from your computer. You can upload TTF, OTF, or WOFF files. Confirm that you own the font and have the right to use it, then click Upload.
The font will now be available in the Brand Kit editor under Font > My Fonts.