Moved to Hardware.
This is a good question. People have certainly re-purposed hardware in the past, so I would assume it's at least plausible. Depending on how the firmware is stored (EEPROM, ROM, etc.) it may or may not be easy to do. If the calculator allows firmware upgrades (like the TI-83+ does) then it should be relatively easy to create your own firmware.
Else, just get an arduino.
Assume that everything I say is or could be a lie.