Check your options sticker to see if your car has it. If not, you may have to ask a Porsche tech if it's something that can be enabled or if extra equipment is required. Either way, it's going to be a minimum $500 job.
It's a lot easier to just add an aftermarket keyless entry, IMO.
http://986forum.com/forums/diy-project-guides/56773-3rd-party-keyless-entry-diy-why-not.html