You can add it but you'll damage the batteries if you don't also up the alternator voltage. Fourteen volt batteries need about 16 volts of charge current in order that they charge properly and completely and in typical car or truck, especially one with PCM controlled voltage regulation, you don't get that much voltage.
Also, if you're not very well versed or you're not paying someone who is, I would advise against upping the battery voltage in your car. There are a lot of electronic control modules and sensors in modern vehicles that either don't operate correctly with voltage that high or, don't operate at all with voltage that high. And if you don't know how to set the system up, you can do thousands of dollars of damage to your vehicle's electrical system in a matter of seconds.