You can do one of two things, use an attenuator with a gain of 0.5 and use half the range of the ADC (need a low impedance load on your signal, this is the cheap solution).
Use a level shifter with an opamp (need to subtrace 2.5V from the 0 to 5V signal and have a negative rail)
Use the LTC2498 which has a 0 to VCC for it's input range.