I just created a cronjob on the server which downloads a json file directly to xibo-library.
Any tips how I can access this in Xibo or is there a better way to do this locally on the server, not via API.
You cannot manually add files to the Xibo CMS. You need to add them via the API.
You could instead host the file on a webserver and reference it from within your webpage/embedded HTML, but then you’ll lose the ability for your content to play back offline.