It’s possible with a named pipe (FIFO). You have to do the volume import separately after VM creation, because qvm-create cannot automatically detect the correct volume size in this case so it has to be passed manually - which is only implemented for qvm-volume import.
Continuing after the qemu-img convert step:
[user@dom0 ~]$ mkfifo /tmp/fifo
[user@dom0 ~]$ qvm-run --pass-io untrusted 'cat "/media/user/externalhd/win10.raw"' > /tmp/fifo
While this^ is still running, open another terminal and:
[user@dom0 ~]$ qvm-create --property=virt_mode=hvm --property=memory=4096 --property=kernel='' --label=red --standalone win10
[user@dom0 ~]$ qvm-run --pass-io untrusted 'stat -c %s "/media/user/externalhd/win10.raw"'
53687091200
[user@dom0 ~]$ qvm-volume import --size=53687091200 win10:root /tmp/fifo
[user@dom0 ~]$ rm /tmp/fifo