As above there are no benefits using your existing dated system.
However, you can buy HA motion detectors to do what you need and they will work flawlessly.
I have a decent alarm system with input output units coming of my alarm system controlling lights for courtesy, to make sure all lights are off when the system is fully or part armed, and of course to switch on lights in sequences when the system goes into alarm to give the appearance that someone is at home.
I do it this way because I know the limitations of my alarm, but its not the way I would recommend a diyer or an experienced HA'er should do it, most experienced HA guys would laugh at the way Ive done it