Hi and welcome to NICO Club!
You could add a whole stand-alone system anywhere you have room in your car, but it's not the most practical solution. Having one set of speakers working together is best for clean sound and having one head unit that can deliver signal to those speakers from your various source units will be least complicated. A loc might not be needed if your amp has low level inputs (most do). Also, you may have the option to split off of an existing signal line that may be RCA pre-amp lines if you have a Bose amp already in there for the factory stuff. Take a look at your existing system in the FSM and see what you have. Here's a link to that:
http://www.nicoclub.com/service-manual? ... 0Altima/av
Basically, you should be able to add a sub, box and amp to your existing system and it should all work fine.