Reprogram a calculator to a cash register
It would be essentially impossible for you to reprogram the microcontroller in a commercial product like a Canon calculator.  First of all, the chip is probably propriety, and not something you could find documentation on.  Even if you could, it is unlikely it is able to be reprogrammed within the device -- the chips are likely preprogrammed at the factory before being soldered in place.  You would need to buy the necessary hardware to reprogram the chip -- that alone would cost more than your printer. 

Then you have the issue of writing new firmware.  Without source code, you would have to write the entire firmware from scratch.  We're not talking a few days, but several weeks, at the least, of effort.  Anyway, without a schematic, and a knowledge of all of the other chips on the board, you would have no idea where to start.

Is it possible?  Very unlikely.  Is it worth the effort?  Absolutely not.