Room Nucleus One USB port out to fiber converter to Ethernet DAV?

You are much better off using the Ethernet and Roon.   I'll be surprised if you can use the Nucleus USB port as, essentially, a USB network adapter port. It’s only there for music, and expects a USB music receiver.   

So, you could, as mentioned, use a USB to coax converter (though this is iffy due to hardware compatability issues.) and end up with S/PDIF, but that loses the asynchronous features.  

Using Roon over Ethernet allows the use of the M33’s asynchronous mode and internal buffer.   Having said that, you can use an isolator or even do Ethernet/fiber/Ethernet conversion if you want to which maintains the asynch features.

Asynch is important because it allows your DAC’s clock to be the single metronome used to reproduce music.  

As @erik_squires said - use M33 as roon end point. You don’t need to use nuckeus usb. Go to Settings, Audio and enable M33 as a device in roon

Thanks again. @erik_squires . What you’ve described (Ethernet/fiber/ethernet) is what I’m doing currently. Just wondering about getting the M33 off the switch.

Probably not doable. Or not advisable.

-K