The original sounder cannot be improved (infact i recon you can fall asleep under the car without it waking you up, its that quiet) you will have to go for an additional alarm and probably end up with two fobs (original and upgrade)
If you 'mess' with the original you will alter the cat1 catagory,so best to have an additional one of your choice as a back up system.
You can still fit what you want as long as the original is not altered to retain your cat1.