The Senseair K30 does have an I2C interface, it's just not documented real well.
The K30 specifications can be found here: K30 | Air quality & Gas sensing technology from Senseair
There is a diagram on Page 4 of that specification that shows where the I2C connections can be found on the PCB. You will also find the K30 I2C App Note there.
The K30 does not have any I2C pull ups onboard. External pull-ups should be to 3.3V, not 5V. The I2C interface operates at 100KHz and uses a request/response interface protocol. If you want to read a register, you must first write a "Read RAM/Read EE" command along with the register address. The response will contain the register read data and the completion status. There should be some example K30 I2C code on GitHub.
Perry L Hunt, Sr. Applications Engineer, AKM & Senseair AB