You can login as Administrator if you enable the user account. You can also just turn admin approval mode to "never ask" in the Control Panel.

While you can't login as SYSTEM (since it's not really a user account), you can trivially open a shell under it's account (and you can do everything you need as an administrator anyway, so there's really no point).

I used to use a PowerShell snippet for adding SeBackupPrivilege and SeRestorePrivilege to an app or explorer window. That will let you read/write anywhere on disk.