For Apple TV (2nd or 3rd generation), the ‘apps’ are loaded from Apple configuration as set for the country or region. Services are available as presented. Users can’t add to that. It is what it is, and no efforts for new services are expected for these models.
The Apple TV box is not a tuner. Apps can stream if served in a compatible format, as send by the content provider. In case it applies, access to ZDF is for users in Germany, not abroad, as their usual licensing contract for most of their programming doesn’t extend beyond the borders. ARD seems accessible to me from outside Germany without warnings about that. There are apps for ARD and ZDF for Apple TV (4th generation or later), as developed by these two.
You may be able to AirPlay from a mobile app on iPhone or iPad to your Apple TV (3rd generation), if the content provider allows for that? Use AirPlay to stream video or mirror the screen of your iPhone or iPad - Apple Support