Umm, no you can't add a 50/60Hz switch to the Mega CD, you CAN add one to the Mega Drive (this mod affects the Mega CD as well, but isn't IN the Mega CD) this is probably what you are reffering to.
Your problem has nothing to do with any 50/60Hz issues though. The problem is actually an internal BIOS check that prevents import games from running.
The check routine doesn't seem to hold to any standard, game code and check routine are inter-mingled, and this is why SCDconv fails alot, it sometimes writes over the game code on some titles.
And sadly, there are no other software conversion utilites than SCDconv that I am aware of, the only way to play foreign games on your MCD is to either get a system converter cartridge (like the ones made by datel) Or, you can fit a Multibios.
When I really started getting into import gaming I took the latter option, and it rocks!
I'm afraid the Mega CD is a complex piece of hardware, and there are no "quick fixes" available.