I'm not too sure what's being asked. su is simply the way one user can login as another whilst logged in themselves, it follows that both require a normal valid entry in /ec/passwd. Giving su - <username> will log you in as the user and will give you their environment, .profile etc. Unless you're su'ing from the root account, however, you'll need to know the user's password.
Hope this helps - if I've misinterpreted, please feel free to post back with additional details as to what you're trying to do.
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