There are few ways to do this, as others have described above. I've got experience with several different methods, so I'll chime in with my findings.

If you're going to just respace a Shimano cassette, DaveSS is right that an 8/9/10 Shimano freehub isn't wide enough, but alcjphil is also correct that if you use an 11-speed hub there is enough room. Also, as ljsense said, you can just drop a cog to make the extra room. I think either of these methods requires some spacer behind the cassette. Shimano's 10-speed cassettes are made to extend beyond the end of the freehub, so you have a little wiggle room in the size of the spacer. I think you need about 1 mm.

The other thing you should be aware of is that you can't replace all of the spacers on a Shimano cassette. On Tiagra cassettes, most of the cogs are riveted together, though you can undo that. The bigger issue is that the three biggest cogs are in a spider with builtin spacing and the two smallest cogs have spacers builtin. In practice, that doesn't matter. As it turns out, if you replace the other 5 spacers with Shimano 9-speed spacers (cheaply available), the spacing will be very close to perfect. The limit screws let you fix the derailleur position for the biggest and smallest cogs dead on, and the rest will work well as long as you're within about 0.5 mm. With the 9-speed spacers, you can get everything within 0.25mm.

The other trick, which Dave Mayer described, is mixing generations of Campagnolo shifters and derailleurs. This works great. I've also had success using a Sachs New Success rear derailleur with Campy 10-speed shifters. One caveat here is that some of the older Campy derailleurs extend a little further in toward the spokes than a Shimano derailleur would so you might get some scraping of the spokes in the big cog when you have the limit screw set wide enough to shift onto that cog. I had this problem with one bike and decided to drop a cog to make more space. Otherwise, it shifts as well as any bike I have.