If you want to add a rain sensor connector, you can use any of the available GPIO pins. Just connect the rain sensor (which has two wires) between teh GPIO and GND. Make sure to set the pin to input, and you may want to add a pull-up resistor if the GPIO pins do not have internal pull-up.
I have tested this on my “Opensprinkler Pi v1.2 + rain sensor” setup and it only works when I push manually the button on the rain sensor. The rain is not detected even on the 3mm level.
When I plug the rain sensor on the pin 8 of the GPIO, does it deliver 24V AC (as requested by the rain sensor) or only 5V DC ?