To completely convert a nes game into optimized 3D form, firstly we should go through the game and create a series of save-state at key points. That's 30% of work.
Save/Load feature is very helpful but for some hard games, it will still take a lot of time to do it. With genie-codes you can greatly speed up the progress. 3DNes for now haven't supported genie code yet but you could use the third-party tool below to inject any genie-code into roms:
Hope this help!