DJM2000 as MIDI Controller for Rekordbox

Yes, but why? It's an audio mixer, and if you assign the controls as MIDI and still have audio going through them, they'll be affected both by MIDI and audio.

To use Rekordbox Video. Therefore I need a Controller for the faders in the software.

I will use internal mixing and assign only the master of the djm

You can't assign the audio output to the master; my recommendation would be to use it in EXTERNAL MIXER mode and then MIDI map the controls you want to the appropriate functions (eg. DJM's crossfader > video crossfader).