Describe your current set up. Impossible to answer that question without that information. However with a DIY you can make nearly whatever on/off signal to work with the level input of GHL.
A picture or brand? I ask because it must be some type of controller that start the pump when the indicator says it should. If you want to use the indicator and the pump fo P4 - you need to use them without the controller. You have to seperate the sensor and the pump from the inbuild controller. Use one of these and cut one of the wires and connect to the two wires of your present sensor.
The pump must be connected to a switchable outlet and that outlet should be controlled by the optical sensor.