Where there's a will, there's a way; but I betting not easily, not the way it folds.

However, fitting a Curl with a Sturmey-Archer 4-, 5- or 8-speed IGH or Shimano 5-, 7-, 8- or 11-speed IGH looks pretty straightforward. In fact, in the picture on Thor's Facebook page the bike is decaled 'Curl i3', hinting perhaps of Dahon-planned future 'i8' (&etc) models.

Certain riders don't like IGHs and have to have a derailleur. Hey, it's all good. There's technical scope, with changes to the right hand dropout, Dahon could offer a Shimano Capreo 1x9-spd derailleur equipped 'Curl D9' at a future point. With, say, a 52T chainwheel, one would get nine reasonably spaced gears in a useful range between 32 and 93 gear inches.