Unfortunately, it is not possible to measure humidity with the TH-03 and an external sensor. The simpilest solution would be to use an RH-02 for the Temperature (0-40 deg C) and Humidity along with the TH-03 for the Temperature (-20 to 80), as you stated. Unfortunately, this has two flaws, this first being that the TH-03 can only measure upto 70 deg C and the second is the wireless modem. Each unit would require its own modem.
An alternative solution, but possibly a little more tricky. Use an ADC-16 this can measure voltage ±2.5 volts, it has 8 channels and is 16 bits plus sign. Therefore you could use thermistors to measure the temperatures and you humidity sensor for the humidity. Problems here you will need to make your own temperature sensors with thermistors. A scaling file will be needed for all the sensor to tuen the voltage into temperature and Humididy.
The ADC-16 can be connected to a radio modem, however you will need to power the ADC-16 externally, either from batteries or via the modem.