Converting NDS to DSIware to CIA?

I wondered if it is somehow possible to convert Nintendo DS (.NDS) files to .CIA. If so, how do we do it? And if not, I'm interested in the technical details (how is DSIware different from the NDS retail game format?).

I suspect it's not possible, cause, from what I see, DSIware games are rather simple when compared to retail NDS games. Also, (to my knowledge) there are no retail games also available as DSIware. Apart from DSIware I see no way (yet) to get .NDS files installed.