UPDATE: YES IT WORKS and even with my newly acquired Focusrite Octopre Dynamic (there was no 8Pre to be found across EU, so I went with Focusrite)
Arturia Support haven't been very responsive and haven't even answered this very simple question - but selecting USB as ADAT source for digital output works just the same way as selecting DAW for AUX output on Studio.
Just select ADAT-Line option on Octopre Dynamic frontpanel and let Studio be the Masterclock for ADAT.
IMPORTANT, it will only work with 2 physical TOSLINK fiber cables - one from Studio "optical out" to Octopre "optical in" and the other cable from Studio "optical in" to Octopre "optical out". Also important is the connections you put the cables into (as you probably need the clock to sync over ADAT) - On Studio use the ones marked with "ADAT 1-8 (1-4)" and on Octopre choose the right (seen from the back) TOSLINK connectors for both in and out (marked 1-2, 1-4, 1-8).
That's it - it works great now and you can enjoy both addition 8 inputs and 8 outputs from your DAW - Ableton Live 11 in my case.
Kudos to Focusrite support, you can actually call them and talk to a real person if you need this - an approach Arturia should add to their support option as well. I probably spend 2-3 min on the phone with Focusrite support, when I wanted conformation that Octopre Dynamic was able to send analog output from ADAT sources. To compare it has taken Arturia support more that 2 weeks and still they haven't send me the above resolution, I had to find this myself.....