[Hellz]

no.. you will need to think this out before you do anything.

first off, you wont do anything with the amp, other than keep it or pull it out.

you can keep it to run your component speakers(which is fine)

pull it and install a real amp to power all speakers/sub

first off, do you still run with the OEM headunit?

If so, you will need an adapter, or a splice from the headunit to the speakers.. this will go to your amp. so if your new amp only accepts RCA, like i said, you need to think this out before you do anything.

i run a pioneer headunit, but retained the stock speakers and amp.
from the headunit, i have another amp hooked up from the pre-out from the HU.

if you are wanting to keep everything stock, but just simply add a amp and sub, you are going to most likely need an AMP that accepts signal from speakers instead of RCA.... but as you guess, you are going to lose a speaker.. most likely a rear one(s)

i would recomend 1 of 2 things...

take to a pro to do this
or
replace the stock HU with a aftermarket one