Greetings and welcome,
Your best bet to hookup an additional sub to complement the existing 8 inch stock would be to tap into the sub channels. The rumor has it that the stock sub crossover point is around 70 hz (cycles). If the aftermarket amp that will be running the additional sub has the ability to be crossover lower, then that will be great to get the deep low bass.
The Alpine premium factory amp is running the built-in Alpine's Euphony Surround Sound processor, the rear door and hatch is running a predetermined delay or time alignment signal. The sound might not be to your liking because it will not be matched up to the front speakers. The front 6x9 and tweeters are set to high pass crossover points and that would rule out get down into the low bass range.
Best for now.