Even if you spent the ungodly amounts of time needed to disassemble and understand the code of an average N64 rom, you'd still need to rewrite the entire thing from scratch for the DS. It's not like you can run it through an application which'll pop out some C-code for you to compile on whatever platform you please. If it was that easy pretty much all games would be multi-platform.
Come to think of it, the entire line of thought baffles me. How's it not possible to realize how hardware specific most applications are in their programming, especially when considering games which usually are the apps to really showcase hw-specifics?