Nissan Altima 2.5SL Premium Bose System?

So I have a 2004 Nissan Altima 2.5SL and it already has a premium system that has a small amplifier and Bose speakers, I'm buying subs and a new amp off of a friend, is it possible to hook all this up to my stock head unit? and do I have to rewire my Bose speakers to my new Amp or could I leave it in the old amp?