Adding system level permission app to an existing android ROM

Lets say I have an existing Android 11 ROM and I would like to add a priv-app app to the system with system level permissions. In order for this app to be able to write secure settings and become a device owner and so on. I would like to keep secure boot enabled. If I understand it correctly these apps need to be signed by the platform manufacturer's key which I do not have obviously. I do not want to use root or some sort of workarounds in magisk, XPosed/LSPosed module or a frida-script. but I can open the system.img and change or add files to it. Is it possible to add your own platform key or replace the existing key and then re-assign sign all current apps with it?