This is something I've been wondering for a long time. Would this actually be theoretically possible? This especially boggled me in regards to Forza, because for example with certain car mods for games (like car mods for GTA V) or car models from Garry's Mod (like TDM Cars or SGM Cars, etc.), they tend to say the model actually came from Forza Motorsport or Forza Horizon. So this got me wondering... if people can port car models over, surely they can port map models over, too? I don't know. I'm just throwing a curveball here. As for The Crew's map, I found a thread here on Zenhax that appears to be some sort of map tool. I didn't look at it all the way through but I'm guessing it lets people mess with the map model or export it or something? Maybe someone here who's got loads more experience compared to me could correct me with this stuff if I'm wrong. Thoughts?
Sadly, those maps are MASSIVE, and running that map as one object would kill your pc. Splitting it up would be a pain, along with importing all the objects on the map. Smaller maps, like those from assetto are definitely importable, though.