AFAIK, the Nucleus only supports HFS+ file systems as read-only. Without jumping through a lot of really nasty hoops, the Mac can neither format nor even mount an already-formatted Ext4 file system.
So, if you go the route you are thinking, you won’t be able to subsequently add new music through Roon. You’ll have to reattach the drive to your Mac to write to it.
Alternatively, since Roon exposes your USB-attached drive as a Samba Share, you can attach the drive to your Nucleus, remote-mount it on your Mac, and just save the music files to it over the network. That’s what I would do.