Hi bueroblanko ,
Yes, it's possible to host FontAwesome locally.
To do this, you simply need to disable "Import Icon Library".
You can find this option by clicking on the gear icon on the bottom left of the dashboard.
Then go to Settings -> Icon Library -> Import Icon Library
Also make sure that you have the correct FontAwesome version selected as to the one you will be hosting locally. Otherwise, the icons on your button will show up as a rectangle.