you just can't fit sensors the alarm system has to monitor that function there is an upgrade to a cat1 alarm available from kia dealers cost inc fitting is aprox £250- 300 kit only is £155 part no FQ09410002
There are 'after-market' ultrasonic devices that can be fitted to any carwith a factory standard alarm. I had one on a Landrover Discovery that had a very sophisticated alarm system. It actuated the alarm by being connected to the bonnet switch and simply made the factory fit alarm think the car was being tampered with. Try google.