Arduino is a huge ecosystem, if something could enable Pi interface adapt to a standard Arduino pinouts, it will be able to access a variety of Arduino expansion boards, to get more available resources. You may need this item- ARPI600. What's more, the ARPI600 also support XBee modules, make it easy to add wireless feature to your great project.
Features
Compatible with Arduino UNO, Leonardo, easy access to a variety of expansion board for Arduino
Onboard XBee interface, easy access to a variety XBee module
Onboard USB TO UART, serial facilitate debugging, can be configured to XBee USB Adapter
Onboard sensor interface for easy access to a variety of sensors
Onboard ADC, 10 bit, 38KSPS, 11 channels (6 channels for Arduino interface, 5 channels for sensors)
Onboard RTC, which is one of the most commonly used functions