Knock detection is tricky. It depends very much on the exact architecture of the doorway and the material of the door. Some people have it working, a lot of people find they get way too many false alarms, even a truck going past can set it off. You can try it and see.
There are several zwave vibration sensors, but the issue isn’t the sensor as much as it’s just whether the knock is a distinct enough vibration. So you tend to have the same problems with all of them.