The OpenSprinkler firmware currently does not support wireless sensors and does not have XBee transceiver. It has a pin header for radio frequency (RF) transmitter, which can be used to send signals to remote power socket (the so called RF station feature). If you need to use wireless moisture sensors, you will need to add a transceiver (depending on what type of wireless signal the sensor operates at) to OpenSprinkler. There are plenty of spare pins (including SPI and I2C) on OpenSprinkler.