XBee Wi-Fi module connection directly to UART Sensor
I've not use the wifi model but i don't think there's much difference in how they work. 

You just need to send the commands through wifi to your xbee module and it will get passed to the sensor. The sensor should then react to the commands and return the data you requested, which will then be transmitted through wifi.

The baudrate of your sensor need to match with the xbee's baudrate for this to work, and also the logic level.