How to add tags to self imported films in Apple TV app on macOS?

f ExiI just discovered the Apple TV.app on my new Mini(2013). Looks like a useful app, but I have a problem:

Apple TV allows to import self recorded films. After import it shows the headers for Cast, Directors, Screenwriters and Producers but I cannot find a way to add this information.

In films bought at Apple these tags were filled already.

I tried to use 'exiftool', but these tags cannot be written if not already available. Exiftool says that these tags are from directory "Plist - XML"... but which Plist?

Has anybody an idea how to complete this information on self imported films?

Regards - Matthias