I have an imac and am the administrator and was wondering if there was a way to allow me to log in to any account on the computer using an administrator password or something similar?

If you are logged in as root, open a terminal and type

su username

where "username" is the user you wish to become. You can also change user's passwords by typing

passwd username

first login inot system via superuser passwd and then used su username command to login into any account.