The question was addressed exactly as OP asked.
Your case is different, since you are not trying to create an iso.
Let’s set an example:
If you have vbox.raw in /home/user on qube1
Create a standalone as normal, say stand1
Set the root size to 55G
cd /dev/qubes_dom0 and identify vm-stand1-root - ls -l will show the
actual device - let’s say it is /dev/dm-100
qvm-run -p qube1 'cat /home/user/vbox.raw' > /dev/dm-100
This will work fine if your VboxImage uses a single disk.