Odd, I didn't run into any problems. Not having an adventuring Curse party, I just loaded up the default party, removed all the characters and exited game, copied the six .GUY files into the Secret\Save directory. Then in Secrets, Add some or all of the six characters without problem. You might try un-equipting all the gear on your toons inside Curse, then Removing them and trying with those .GUY files.
If that fails, you could try installing the Wizardworks version of Secrets, setting it up and running it to make the save game dir, and then try with that version. I've never seen the Gamefest version, so don't know anything about it. I looked over the WW quick start booklet, and there's nothing useful in it that would help with this. It's just a very general dos guide, on how to do what you are already doing much easier with a modern OS. That's all I can think of trying at the moment.