Of course you can copy a savegame file to an USB drive and then paste it at the new Account, but I don’t know if it would really work, as some things probably are bound to an account… Challenges, trophies, DLC-stuff.
I just read that it should be possible to transfer a savegame between Xbox (PC) and Steam. Some wrote that it worked, so why shouldn’t it work on the same system with different accounts? Take a try.