It depends
If you have the optical Audio system - no. If you have a standard looking amp with a big fat black connector you might be able to reuse it if you can figure out the wiring inputs.
Only Crutchfield sells a connector for the stock amp however it's been out of stock for over a year so don't hold your breath waiting.
The stock amp does not have RCA inputs so that will be your biggest challenge.
My recommendation is to buy a new amp on ebay.
For under $100 you can get a 4 channel amp that can run the dash speakers and a set of subs. Here's what I've got in my Boxster and it's plenty of power:
http://cgi.ebay.com/PYRAMID-ARCTIC-2000w-4-CHANNEL-CAR-AUDIO-AMPLIFIER-AMPS_W0QQitemZ170239226078QQcmdZViewItem?hash=item 170239226078&_trksid=p3286.m14.l1318
Good luck,
-Steve