I'm afraid your only option to get that working would be to modify the source. There is a lot of math and coding involved in what you're wanting to do. Namely, finding the proper bullet trajectory based on the cursor's position onscreen is a task in itself, while moving the crosshair around the screen seems simple, there is much to be thought of on the backend of things.
The one and only,
Only those who sow the seeds of their desires will reap their benefits later.
However, I have seeds of my own to tend to. I don't have time to be someone else's watering can.