If I remember correctly, I think that importing someone else's save would result in the game wiping its save data, since it can tell if it's being run from a different console.
I could be wrong, though. The only thing I can think of is finding the part of the save file that contains the flags for unlocked course elements, but there doesn't seem to be a save editor for Mario Maker 3DS other than the course importing homebrew tools (like OCDM).