Yeah it would take a lot of effort sure but that is exactly how video games start out. They start with concept > then storyboard panels > then character design. With this type of game mode creation the modders (or creators) already know everything including the story (supposed story that is).
All in all wouldn't be nearly the challenge as if they were doing this without any of the above so def possible if unlikely
Yea... Like they modded Black Mesa Source. Full on remake of HL1 in HL2 engine. Masterpiece... And then they went and rerecorded all of the speech in some basement, instead of using original sound from HL1. I still can't decide if i was more disappointed when i run BMS, or at the end of Mass Effect 3.