for storing large amounts of data, with somewhat slow access speeds, it’s pretty easy to interface an SD card or micro-SD to themicroprocessor via the SPI interface. Then a FAT16 or FAT32 file system - available freeware.
But alas, no can do for program memory, to my knowledge. filling up 64KB or 128KB with code rather than in-flash constant data is a LOT of code-writing! If you’re in that world, maybe you should be using an ARM7 or ARM Cortex M3 like the mbed. I’ve seen an Arduino-form-factor board using these ARM processors.