Good luck to you.
But, I'd be very leery about adding this to the alarm system. The alarm system, in conjunction with the DME, controls so many of the car's functions that if you're not 100% sure of what you're doing, you could seriously screw things up. I mean to the tune of mega $ to get it right, esp. if you somehow damamge the alarm module. Are there Boxster specific installation instructions included to wire it to the alarm module?
Our cars are not high on the list of 'most stolen cars' - about 20 times less likely to be stolen than a honda s2k for example. Unless I regularly parked overnight on the street in East St. Louis (and if I did, I wouldn't have a Boxster), I think it's overkill.
If you're worried about someone breaking into it, don't leave anything valuable lying visible to the outside.
But, if you decide to do it, let us know how it turns out.