You can't just write a driver to add active stylus to any touch laptop.
How the active stylus works depends on the touchscreen manufacturer. But in a typical capacitive touchscreen that supports an active pen the stylus transmits a signal from the tip that is detected by the touchscreen controller. The signal is encoded with pressure and button information.
There are some bluetooth active pens that "work" with apple products, but they're flaky. Those devices listen for the touch signals, then drive an inverse signal through the pen tip to make it look like a regular touch. In this way, there's no way for the computer to know which touch is from the pen and which is from a finger.