You'll probably find the complete wiring looms are different between alarm equipped models and those without, not to mention usually built in PSA alarms are annoying and to be blunt not terribly reliable.
A decent (professionally fitted, so you can show the details to your insurance people) aftermarket unit will be far superior and could be made easily enough to operate the central locking if required.
Andy.
91 205D-Turbo, gone but still missed
02 106D, TUD5B, gone but not really missed apart from the MPG