wireless I2C/SPI communication over XBEE with multiple slaves
Some of the Xbees are programmable, so you could in theory program one up to be an I2C or SPI remote.  I don't know what the toolchain is like though; that might be a laborious process.  Same goes for ESP8266, but that's relatively easy to use now.

Otherwise, you will need to put a little microcontroller between each radio and its local sensors.  That micro is responsible for polling the sensors at the appropriate rate and sending measurement data back to the central collection point through the radio.

Since you seem familiar with Arduino, you could perhaps consider using an ATtiny85 and nRF24L01+ (if it has sufficient range) at each location, with another nRF24L01+ at the central arduino; there are free libraries that do everything you need.  Or use an Xbee Pro (900MHz band) if you want miles of range, but the bitrate will be ~100x lower.

Your radios do not need to support the raw I2C/SPI bitrate, they need to support only just enough to convey the necessary information from your sensors.  If you need samples at 10kHz then that's really difficult, but if you need a number from each site only every hour or so, then that's really easy even with the slowest radios.