For that purpoes I am trying to attach the USB network card to the sys-net.
That will likely not work. More complex USB devices often have issues
when used via USB proxy. What will work is if you combine sys-net and
sys-usb so that sys-net has access to the USB controller and fulfills
the function of sys-usb.
If your keyboard/mouse connect only via USB then be very very careful
what you do as to not lock yourself out!