Can android user become a super user in android?
Depends what you want to do. With su (if it is installed) you became a superuser... But if you want to access some files you actually need to be the user system.  This looks like it is your case. The credentials of user system can be given to an app (apk) if you are the owner of the android os image or you have access to the private keys the os was built with... If you do then you need to sign your app with those keys and mark it in manifest as user system...