See
http://www.stanford.edu/~dru/tripleize.html
http://www.stanford.edu/~dru/triplecheat.html
If it was me - I would get the triplizer and install a 30T granny on your existing Campagnolo crank.
You may have to hunt around for a used triplizer (and try various spellings) but anything is eventually available on eBay. If none is available, Campagnolo triple cranks are all over eBay.
If you need a longer BB spindle, Campagnolo made 111mm ISO square taper BBs for triples, you can find them on eBay.
See if the FD and RD work, if not then Campagnolo triple derailleurs are also on eBay for not crazy sums in the mid-grades (Centaur, Veloce, Race Triple). I personally would try to use your existing RD, use a chain long enough to permit big-big without damage, and simply don't ever use small-small (i.e. only shift into the granny when you're already in the biggest cog) - if you can make that work, great.
In general, I think it would be a shame to be putting Suntour stuff or mountain bike derailleurs on a De Rosa.