I guess you are referring to just the speaker than sounds the alarm. If that is the case then , it's not possible to just add an alarm to existing OEM
central locking. Even I am looking for the same, and when inquired TOYOTA dealer, he said entire OEM
central locking would be changed and will cost 6000 +. when I inquired the aftermarket accessories shop, same answer, that I'd have to go for a complete CL change, adding just an alarm is not possible. Not sure though, how much truth is there in this.