The reason you don't add sorbate to an unclear wine is because there are too many yeast cells present for it to work properly. The over- abundance of yeast cells will overwelm the sorbate. All white papers on sorbate do a good job of explaining this, but I'm sure many people have never researched sorbate. Many people add sorbate to wine shortly after they get it over in the secondary saying that they are "stabilizing" the wine. Well, nothing is further from the truth.
For sorbate to prevent refermentation when you are backsweetening, you must first have the bulk of the yeast cells removed thru racking. Otherwise, there are so many yeast cells that the sorbate can't fully work and you will have a very slow re-fermentation going on. And bulk aging is the only tactic that stabilizes wine because the sediment that falls out of a wine is all the unstable components. So getting these off the wine is what gives stability.
The only time you add sorbate before clearing is with a kit. But that is the only time you should do it. For non-kit wines--don't sorbate until you have the wine clear.
weaver--If you let the carboy sit until the sorbate is hydrated, then stir, it should go into solution. Go to Bed Bath and Beyond and get yourself a mortor and pestle and crush that sorbate up!! It dissolves so much better--or put it in some water first. Whatever works for you is fine. But just tossed into the carboy whole---well, they don't like going into solution in a cool liquid like wine.