I wouldn't.
I'd get an aftermarket HU (which wouldn't need a 4ch amp for a moderate system), and a new sub/amp.
Basically there isn't a point adding to the stock system because you'll still have the stock limitations in the end. It's better to do it right once than do it in pieces and not be happy and spend more money.
But, if you want to replace the stock amp for the 4 corners all you'd need to do is go into the passenger interior panel in the cargo area and line-in the stock speaker wires into the new amp, and run new power and ground wires, and splice into the stock remote wire. Your stock system has seperate amps: 4 + 1.