Yeah, it's possible, but why would you want to? Most comments I've heard from people with it, hate it. Anyhow, to your question, you'll need the amp, the batwing controller and the wiring harness that runs from the amp back to the batwing. You'll have to run those both thru the firewall and the trunk wall in front of the battery. Plus I think a few more wires on top of that...
Sounds like your goal is better sound. If so, check out these two links on the wiki:
Upgrading the Boxster's Sound System (by Andy M on Renntech)
http://boxster.wikia.com/images/b/b6/Boxster_speakers_update_article.pdf
Door Speaker Baffle Template (by some jerk named Burg Boxster)
http://boxster.wikia.com/images/b/bf/Door_Baffle_Template.pdf
The biggest weakness in the sound system is crappy speakers and Porsche outdid themselves on that task. The improvements you'll acheive by swapping out the OEMs w/ some decent (under $100 per pair) speaks will be like night and day.
Good luck!