Check with Gaerlan, he does lots of aftermarket Dahon work. There's also a guy on BF named Brakemeister who does the same at his shop.
But offhand, I'm going to say it can't be done, or at least can't be done well. If it could, Dahon would've put a double on a bike like the Speed Pro / Speed TT rather than use the SRAM DualDrive.
FWIW, some other folding bikes are more amenable to front derailleurs (but are still uncommon), e.g. Bike Friday and Swift, maybe the Airnimals. Several Moulton separatable bikes also use FD's.
None are cheap. So it's possible that swapping the cassette and/or front ring may be the most economical option.
Last edited by Bacciagalupe; 01-21-07 at 04:51 PM.