So that I can hopefully fully answer your question, could you please provide more information.
Are you wanting to add a custom font so that it is selectable in the Typography section of the customizer?
If so, is it a Google font that is not currently in the available options?
Or is it a font that you have the files of, and want to add it in the mix with the Google fonts options?
Or do you want to override the theme fonts with your own font?
Do you have the files (.ttf .woff. eot etc..) for the font you wish to use?