Here’s how to create an admin account without knowing the current administrator password.
This is nothing new to people familiar with unix, and can’t really be
classified as a “hack”, but it’s an easy way to make an admin account
without a password. This particular method is used if you, for some
reason, can’t get into your account at all, for example: you
lost MAC Admin password
and you account is protected with FileVault. This process basically
forces your computer to re-run setup, which is what you see when you
setup a new Mac.
This process will take about 5-10 minutes. If you have a password,
or WEP code for your network or internet, be sure to have that at hand.
Source: http://www.macpassword.com/mac-admin-hack.html