It would void the warranty. Relax, Honda City is low on priority of car thieves. I would suggest comprehensive insurance from a good company, a good steering crook lock, tint paper on the rear quarter glasses (makes it harder for thieves to break this window to open door locks).
A determined thief will steal your car no matter which alarm system you have. Your job is to make it hard for him to steal. My car doesn't have an alarm system and a car mechanic told me it's harder to get into a car without alarm as in alarm fitted cars they just open the bonnet and ground/short the system to automatically open the door locks 