The hard part is not running things off a secondary battery, the hard(er) part is charging the secondary battery.

You can easily add a second battery, and connect things to it as you would the main battery. You will, however, have to find a why to charge the new battery. It has to get connected to the alternator somehow. Battery isolator or solenoid is a good way to do it.

Make sure your alternator is up to par also....