I suspec you can not. Here is what I just saw:
try to load a saved game. I see the ones I have and select the up level. I see PTW stuff and saves. I select up again and see I am in CivIII, but I do not see my save folder. I know I have them, so I am guessing it does not want me to be able to load them.
I was able to load an old deity game form PTW.