Back Sweetening a Cider
You don't want to add the sorbate until the fermentation is very complete, i.e., clear and stable. Typically added AFTER the beverage has been sulfited. If there is any renewed fermentation with the sorbate present in the beverage you typically get an aroma and flavor of fresh cut geranium flowers (really the character of the broken stem) -- very unpleasant.
So make sure your cider is clear, rack off any sediment, and then treat it with sulfites. Once it is obvious that fermentation is not restarting you can sweeten and add the sorbate at the same time.