How can I add a powered subwoofer to my existing home theater if I already have a passive sub?

If your subwoofer is a passive sub then this will more than likely be an easy fix. Take a look at the system - most passive subwoofers are connected with the outputs from the front speakers (meaning that the speaker wire outputs for the front speakers are run into the subwoofer, and then from the subwoofer to the front speakers). If this is the case then you can easily put in a powered subwoofer the exact same way, but because it is powered you will have control over it through the subwoofer itself. However, if your receiver happens to have a Subwoofer pre-out then all you have to do is get a subwoofer cable and connect the subwoofer via that cable, and you will have a true powered subwoofer, and could even keep the existing subwoofer in place as well.