If you are talking about the first time login account setup, then see this script:
https://forums.developer.nvidia.com/t/jetson-nano-all-usb-ports-suddenly-stopped-working/75784/37
…direct link:
https://forums.developer.nvidia.com/uploads/short-url/xMF6uijxdcBfNYWQxDNcQEWEmOJ.gz
What this does is create accounts on the “Linux_for_Tegra/rootfs/” content before any image is generated. See if that covers enough of what you are looking for, and if not, maybe more could be worked out.