I've done this before. In the future, if you can't get Campy cables, file the sides of a normal/Shimano derailleur cable head down until it slides in and out of the shifter easily (test by sticking the cable in from the front, not from the normal side).

If you look at the spot where the cable head sits, it should be a cup like shape (filled by the cable head) with a small dip in the rim on the outside. Put a dental type tool there (I have some screwdriver which is simply a pointy tip, I think it's some electrical thing), stick the tip of it as far down the side of the cable head as possible and "lever" the head out just a bit. Repeat. It may help to drill a little hole in the side of the head (so the pointy thing has some traction), but I found that it isn't necessary all the time.

Good luck,
cdr