You must have QEMU in that case. What @mdegans mentioned is part of the requirement for installing QEMU.
There is an optional script which can create the user name/pass and set time zone from the PC by modifying the image to be flashed prior to actual flash (performed on the PC side). This script is human readable, and will demonstrate how to set up QEMU and use it. You could edit this script and experiment with that as a way to learn how to do this. See the script here:
https://forums.developer.nvidia.com/t/jetson-nano-all-usb-ports-suddenly-stopped-working/75784/37
More info (also thanks to @mdegans):
https://forums.developer.nvidia.com/t/tx2i-modify-rootfs-to-create-new-user-during-flashing/118076/5
EDIT: Note that you can often perform those script lines by hand on the command line as well.