You can upload via the API. If you’re of a technical nature there’s a working Python command line upload tool here:
http://bazaar.launchpad.net/~alexharrington/xibo/140-rc1-upload/files/head:/apps/MediaUpload/
The GUI version was never implemented so don’t expect that to work.
For details of how to get it setup/configured you can work from the guide on API access which uses the same Python libraries to connect to the CMS:
Other than that no it’s not possible to add files to the CMS by FTP or any other means. Xibo doesn’t run well on shared hosting, precisely because you often can’t make the changes required to support it, or because it consumes too many resources and the hosting provider disables your account as a result.