Is it possible to integrate your own fonts using Webfonts?

In theory, this is possible if you upload your own HTML file. However, we tend to advise against using your own fonts as they are not so widely supported by all email clients, so consistent display cannot be guaranteed. 

It is not yet possible to upload HTML files - however, this function will be available in the future. 

It is also possible to set a fall-back font, so using your own fonts could certainly be an option.