Lots of recipes with simple sugars advocate adding it after the ferment is established. I think the reason is that the simple sugars are easier for the yeast to eat and giving it to them early on makes them "lazy". I'd boil the dex with a small quantity of water, decant into a sanitized container (I use a Pyrex jug) cover with glad wrap and cool. Add this to the fermenter, taking care not to aerate. In terms if when, I'd say after the bulk of the ferment has finished, but before it reaches FG.