To be honest i just asked you what your aim for the whole task is. (You didn't described it in the first post. Now you've done it.)
The information i gave should only have told you that its not in the focus of OBS. This is an OBS forum.
As you already mentioned by yourself that ffmpeg could/should work as the bridge for what you aim.
But you asked how "this option to show" in OBS. Hence it's not part of OBS there is no option for that available.
The only way to get it work are indeed custom commands (or better options) to ffmpeg, i think.
So please try aunt Guugle with words like "ffmpeg dnxhr".
It leads to ressources like this:
http://macilatthefront.blogspot.com/2018/12/tutorial-using-ffmpeg-for-dnxhddnxhr.html
Please look if that helps you further...
As others tell too:
We (and all possible successors) would be glad if you find a working solution/combination of ffmpeg parameters, please return and post it here. Thx! :-)