You can use the rain sensor port for sure — a rain sensor itself is a switch (i.e. activated or deactivated by rain). You can connect a normal switch to rain sensor port, and when the switch is closed the OSPi will stop watering. However, the firmware currently does not support any ‘suspend’ function — if a program is supposed to run while the switch is on, the program run will be skipped and will not be kept in the queue. Hopefully the chances of your pool filling overlapping with sprinkler run are small.