'90s Colnago C-97 Decor, 2010 Pinarello FP2, 2002 Cannondale R2000 Si Saeco Team Replica, 1989 Atala Cromor Triomphe, 2000 Giant Boulder SE
I know this reply is WAY late, but I wanted to put this information out there for anyone that might need it.
I was attempting to convert a Campy 8 speed freehub to a 9 speed and became aware through this forum and others of the issues involved with the different arrangement of splines on the newer freehubs. I didn't want to attempt the complicated rebuilds some brave folks suggested. After thinking about it a while, I came up with an idea to add a single 8 speed cog of 16t to my 13-14-15-17-19-21-23-26 cassette, and then use 9 speed spacers in between to reduce the width appropriately. All went fine until I found that the last cog sits almost flush with the locknut on the end of the axle. This next step isn't perfect, but works like a charm - I removed the lock nut and slipped a very thin washer/spacer (1 mm) behind it. I then fit the wheel onto the Bianchi I am restoring to test it out and the cassette arrangement works perfectly with the 9 speed Campagnolo derailleurs I have on the bike!