First modify your Hexabus board to use 3.3V, but completely remove R22 and R24 (see here). Now solder the HYT321's SDA pin to pin 20 (R24) on the ATmega and the SCL pin to pin 19 (R22). Connect the HYT321 to VCC and Ground and solder 4,7k pull-up resistors between SDA and VCC and SCL and VCC.
In hexabus_config.h set I2C_ENABLE and HUMIDITY_ENABLE to 1.
If you also want to use the HYT321's temperature sensor, set TEMPERATURE_ENABLE to 1 and TEMPERATURE_SENSOR to 1.
Humidity can now be read via EID 5 and temperature via EID 3.