The reason you can't do this is because the meters measure pH using a calculation based on offset and slope. When you calibrate it, it measures the mV reading between the probes at the specified pH. It then performs a calculation of mV it rads in the fluid you are testing against the calibration mV. You can't just take a miscalibrated pH meter and add to that miscalibration. It's also worth noting that pH readings are logarithmic so every full number. For example, a pH of 6.0 is tens times as acideic as pH 7.0.