Funny how only pieces of answers are given when the question "Can I put XXX volts on an I/O pin?"
True, the spec sheet says:
From the datasheet:
Voltage on any Pin except RESET
with respect to Ground ................................-0.5V to VCC+0.5V
So consider your VCC source. If using Vin, consider how stable your supply is and what it's ACTUAL maximum will be. When using USB, realize that USB's spec allows Vusb to be between: 4.75V and 5.25V. Keep in mind there is an on-board fuse (which nominally should be 0ohms, but could drop a small amount of voltage.)
So, if powered by Vusb there is the potential for VCC to drop down to 4.75V which means the Maximum to any I/O pin is now 4.75V+0.5V = 5.25V.
Will it burn the ADC? No. If anything did get taken out, it might be that particular input of the MUX going into the ADC.