I was wondering about this question for a while(and I might not be the only one), because it seems possible, but at the same time it could turn out disastrous. This question being "Can I import a game's code to an NES game if it is written in a NES-compatible language(ex: ASM)?"

I'm asking this question because like I said, it seems like a possible task, and I could probably experiment with that for a while before making games from scratch for NES or even Game Boy. Has anyone else wondered about/tried to do this to see if it would work?

The only place you're going to find "NES-compatible" assembly code is in the source of other NES games. Modifying someone else's assembly code tends to be harder than writing your own, especially if you don't have the original source and are relying on a disassembly.

Language or similarity of CPUs does not really matter. All game code is designed and written around the platform hardware, its limitations, and quirks. So the only thing you can 'import' is code that does not rely on the hardware at all, i.e. some game logic. And even with that you can get issues, because NES code always rely on 2048 bytes of RAM available, while Atari 2600 can only have 128 bytes, thus totally different algorithms and approaches used to design game logic as well.