The underlying microcontroller in the VEX Cortex is some sort of STM32F1 or STM32F2. I'm not sure which model exactly (just that it's a ST-made Cortex M3), but they all do have digital I/O capabilities on the pins used for analog inputs. Given the bare microcontroller, you could do so.
However, the VEX Cortex is not the bare microcontroller. According to the documentation, the controller board has been designed to add a much higher input impedance to the analog inputs which would likely interfere with digital operation:
Input Impedance: Analog - The 8 analog inputs each consist of a 470K pull-up to +5V, a series resistance of 10K and a 20K pull-down resistance to the uP.
You can see the difference in the circuit connections on page 13 of the User's Guide (PDF).