I don’t see why you cannot use your proximity detector, but you will need to do some interfacing.
What you’ll need to do is supply it with the 5 V available on the RJ45 connector on pin 2. However, it’s not safe to connect the output directly to the pulse input, you’ll need a pair of resistors to divide the voltage down to 3.3 V. You’ll need a 4.7 kΩ connected between the sensor output and the pulse input (pin 6) and a 10 kΩ resistor between pin 6 and GND (pin 5).
If you prefer to use the RPi’s GPIO, leaving the optical sensor connected, information (naturally for the optical pulse sensor) is here: Directly connecting to Optical Pulse Counter with RPi? - #2 by pb66