Xbox mod scene, MaxConsole, has posted details on a trainer for Mortal Kombat: Deception on a modified Xbox that makes the classic characters encountered in Konquest playable in arcade mode. The trainer has been released in this thread.
However, don't get your hopes up believing that these characters are hidden somewhere in the game. When you select a classic character and an opponent performs a fatality or stage fatality on your character, the game crashes. The explaination is that there are no death animations created for these characters, which results in the conclusion that these characters were not ment for use outside of Konquest.