I think you'll want to add in the lactose come bottling time. I actually need to do the same. The second batch I made was an experimental one using brown sugar instead of dextrose and adding some holiday spices. This was prior to me discovering the fact that molasses adds a very tart flavor. Now I need to back-sweeten.
My only concern is how much lactose to use. 1 lb. per 5 gallons? Or should I just do the add, stir, and taste method? It's pretty damn tart right now...it'll pucker your lips a bit. I just don't want the final product to be too sweet.