Support for magnetic switch has already been added a while back:
In Options->Basic tab, Sensor Type, you can see options for switch sensor. It should be wired to GPIO4 and GND. Please note that in some early versions of OpenGarage, GPIO4 and GPIO5 are mistakenly reversed. So in case GPIO4 doesn’t work, try GPIO5. These pins are located above the distance sensor, marked GP4 and GP5.
If you want to add other sensors, you will need to modify the firmware to support those.