I want to connect the K30 to the same I2C interface as the other sensors.
That's not possible as it doesn't have an I2C interface as I already wrote. That sensor has a serial interface that uses the ModBus protocol but on CMOS logical levels, so you need a level converter for that.
The UNO is a 5V Arduino. The standard Wire library activates the internal pull-up resistors, so the I2C lines are pulled to 5V. With external resistors you might get that to a lower level (about 3.5V) but that might be already to high for some sensors. So it might be necessary to patch the Wire library to deactivate the internal pull-ups.