You don't add sorbate until it's finished, as sorbate doesn't kill yeast. What it does is stop yeast from reproducing- and the yeast doesn't need to reproduce during an active fermentation. In other words, you won't stop fermentation by adding it. It should keep going, but you may have stressed it out. Let it sit for a while, until clear, and then check the SG at that time to see if it kept going or if it did stop.
Next time, let it ferment out and then stabilize when clear (with the sorbate and campden). Then you can sweeten to taste.
Also, rack to small carboys ASAP- at this stage you want only about 1.5 inches of headspace.