Wrong section I suppose..
But told that you could amplify your signal if you need or (better solution) find a 3.3V ADC pin to pin compatible with yours.
In each case you should change some HW on the board.
with ADC0804, by lowering to half of refrence voltage, i get full resolution at 0-2.5v at input to adc in one of my project.
So try to chnage the refrence volatge and get the results.