You can buy 11sp cassettes that have been modified to fit 10sp freehubs. There's s chap on the Time Trialling Forum who does it - let me find a link...
It's 1.85mm to file off, or 0.85mm if your current cassette uses the 1mm spacer as you can just bin that. It's better to take it off the cassette than the freehub if you can, and better still to get Woolie (see above) to do it for you.
I have a hope pro 3 rear, man in bike shop told me that I can just file a bit off the free hub to fit an 11 speed. Is he right?