I don't think you can do this, for two reasons that I detail below:
The PIC controllers typically store their program in flash memory. You can re-program the flash at run-time, but it's not that easy and typically you cannot write a single word but rather must write a whole "page" (or "sector" or something, i.e. a collection of perhaps 64 to 512 bytes or so) at once. This makes it very impractical to modify code at the level you're trying.
Furthermore, the program memory is generally not directly addressable due to the controllers using a Harvard architecture. This means you can't even have a regular pointer pointing at an instruction, because data and code are in different address spaces.