Logging in as non-privileged user
I know it isn't possible to just login as one, but is there a simple way to switch a non-privileged user to a privileged user? I want to be able to take an account like httpd or mysql or nobody and be able to login under that account. Don't tell me that it's a bad idea or whatever, this is somewhat hypothetical and I normally wouldn't do this. I want this to be done with making minimal changes to the passwd and shadow files. So setting the default shell, home directory, and password should be avoided if possible.