About memory expansion with SD/MMC cards

Hi, I would like to know if it's possible to store the Arduino application code in the SD/MMC card instead of internal flash.

Yes, of course you can. It's just a file. You could store it on punch-tape if you want to.

Did you mean "...and can I execute it from there?" ? In which case, the answer is "no".

Not directly - this question pops up all the time. Consensus seems to be that one can run an interpreter of sorts, or with more difficulty transfer the code to flash, and do equivalent of software reboot to run it.