How to Read ADC value without Common GND ? (Possible or not !)
The solution is to always use at least 2 electrodes. You need 1 electrode, that you connect to ground. You can put this electrode to somewhere neutral (this instructable suggest connecting it to your leg, but I don't know, if that position is really good).

An ADC measures voltage and voltage is the difference in potential between 2 points, so to measure voltage you by definition need 2 points/connections.



Note: The voltages from the EEG electrodes are normally very low, so that they completely vanish in the noise. To really get something useful, you need to amplify the signal. I think there are many tutorials out there, that explain how to do this.