No, the dragon form was in from the early design, at least as a fallback if players avoided the initial encounter at the Fort Joy gate, came back later and were able to defeat Dallis or Alexandar (IIRC the designers wanted something more than just teleporting away). It was just dropping a shapeshifting mask that was never implemented (it would only be available very late in the game, and then only if choosing dialogue options so Dallis was hostile in the final fight).
Dallis had plenty of time in charge of an organisation doing research into magic, source and powerful artefacts, so even if she didn't know how to shapeshift into a dragon as an eternal, could reasonably have discovered this during the course of her other activities.
