Many people are happy enough running unmodified Shimano 9 speed cassette. The error is an over-shift not an under-shift so reaching the big cog won't be a problem.
The error will be 0.8mm off arriving at the big cog and 0.6mm at the second smallest (with the limit screw providing perfect alignment on the small cog).
If you want it to run perfectly -
Wheels Manufacturing will convert an Ultegra or Dura-Ace 9 speed cassette for Campagnolo 9 or 10 speed use with perfect spacing. A couple years ago it was $50 + shipping.
The cassette must have the final cogs on a carrier (the Campagnolo 9/10/11 freehub body is wider than the Shimano 8/9/10, so they machine the final carrier so it can hang of the end) so they can't convert a 13-23 where the 23 is loose.
The 10 speed conversion requires a 12 starting cog and ends up with an 11.
Last edited by Drew Eckhardt; 02-24-13 at 12:22 PM.