for egsample if you have a cleric and make a "fire lizzard" "class" , in the game you will have a CLERIC only with changed skills (you can do that with any MM7-savegame editor BTW)A bit Off- topic:
you cannot add any new classes to MM games unless you rewrite theyr code and add new stuff by that way. I assure you, that this "Class changer" has nothing to do with adding new things to the game. It is just another Savegame editor , as i mentioned earlier.
Sorry for any mistakes i made, i didn't have much time to write this 
merry Christmas (that are over now) and Happy new year!