No, they can't install the factory system aftermarket. I added a LoJack for $499 in lieu of the factory alarm I forgot to configure. It slips inside the battery compartment. No switches, its not seen at all. It doesn't honk the horn when bothered, but it does get your car back (hopefully) if someone carries it off. Gets the insurance discount, too.
I'm happy that at least the panic alarm is still there, and that a little light flashes on the lock button when the car is unattended. It looks like there's something "armed." Hope that works!