Yes you can. On the schematic here:
https://github.com/rayshobby/opensprinkler/tree/master/OpenSprinkler%20Pi/hardware
and also the firmware code itself, you can find out the GPIO pins that are connected to the rain sensor and RF transmitter. To connect a rain sensor, hook up your rain sensor wires to the rain sensor GPIO and ground (the additional resistors and TVS diode you see on the schematic are for circuit protection purpose). To connect RF transmitter, connect +5V, GND and RF data pin to your RF transmitter.