Yes, using PSoC1 you can measure a signal ranging in microvolts. For measuring such a low level of signal it has to be first amplified. This can be accomplished using multiple stages of PGAs or INSAMP followed by PGA. But the problem is that the offset at the input of the INSAMP or the PGA will also be amplified along with the signal.
Correlated Double Sampling can be used for taking care of this offset error. This technique is explained in the application note AN2226 and has been implemented as a part of an example project "Interfacing a Pressure Sensor - MPXM2102A" for measuring a differenial signal of 2uV.
Gain error is another contributor to the errors in measurement. This can be taken care of by applying a known voltage, measure the result and store a scale factor of Volts / Counts. Then during normal measurement, this scale factor can be applied to the measurement. For example, apply a known input of 10mV to the input and measure the ADC counts. Now calculate Scale Factor
Scale Factor = 1.0mV / ADC Counts
Store this value in the E2PROM. During normal measurement use the below formula to calculate the actual input,
Input = ADC Counts * Scale Factor
By implementing the above offset and gain calibration, signals in the range of microvolts may be measured very accurately.