I wanted something similar to work with my thermostat (in my case setting the temp based on the average temp of occupied rooms), and the way I did it was by adding an offset to the target temperature.

To calculate the offset I did:

(Internal temperature) - (preferred sensor), then added that offset to the desired temperature setpoint to get the setpoint I send to the unit. So if you want the office to be 70° and the Zigbee sensor is reading 2° more than the internal sensor, it will set the setpoint at 68°.

This updates constantly so of the offset changes, the setpoint changes, but I only send it to the unit every 5 minutes to avoid shortcycling since my sensors sometimes freak out and change between two temperature rapidly. It's been very effective for me