You could consider one of these, or take apart perhaps (not sure of ability of using any possible cogs with either cassette) and mix with your current cassette. Maybe others can chime in on compatibility?
http://www.nashbar.com/bikes/Product...IaAt_fEALw_wcB
Eg. from the 6600 14-25 cassette, use these 6: 14/15/16/17/19/21
From your current cassette, use the bottom 4 - ie. 24/28/32/36.
You'd still have the big gaps at the low end though. If you don't need the 36-tooth, maybe(?) possible to use the 23 and 25 also from 6600 cassette , and then only use the 28 and 32 from your current cassette for a 14-32 combo.