TX2i : Modify rootfs to create new user during flashing

You can chroot/proot into the rootfs provided by nvidia before running flash.sh and do that, yes. You can also update the system and install software provided you modify the apt sources appropriately.

Assuming you want to add a user to just run an a dedicated app, you probably want to use the --system option. Otherwise the user will be visible at the login screen. Nvidia also provides a script to set the default user if you want a bypass it’s creation at first boot.

I see linuxdev has linked to it above. It is linked to in the Nano FAQ as well but is crossed out for some reason. As such it may be advisable to ask in that thread before running it. It may be it’s not compatible with the latest release since Nvidia has made some recent changes to the first boot scripts.