Not a problem to set Aref to EXTERNAL, and connect the Aref pin to the 3.3volt pin.
(must do both to not damage the MCU).
As long as you also power the thermistor/resistor from the same 3.3volt pin.
As jremington said, measure the thermistor at 25C (nominal value), and if possible at 100C.
It seems you have a 100k thermistor.
Leo..