The wiring harness might already be there since TPMS was supposed to come out earlier than it did. If the car was built after TPMS became available, there's a high probability the wiring harness is the same and therefore TPMS could in theory be a retrofit at reasonable cost (unless Porsche refuses to allow it). Wait..., did I say 'reasonable?' LOL, the only thing you might have going for you is the wiring harness is already in place. The cost for a dealer retrofit is likely to be obscene if not pornographic.

There are supposedly aftermarket TPMS systems, somebody mentioned them here earlier.