Can I sweeten my wine when I rack it into the secondary vessel?

Before you sweeten your wine, it is important that you wait until the fermentation has completed, and the wine has had plenty of time to clear out all the wine yeast. Quite often, this does not happen by the time you do the second racking.

Actually, the best time to sweeten a wine is right before bottling. This gives plenty of time for everything to settle out. There is no upside to sweetening the wine before this, only a potential for problems.

The reason clearing the wine is so important is because the wine must become stable before sweetening, otherwise all the new sugars will end up as fodder for a renewed fermentation.

Cloudiness in a wine usually indicates it still has excessive wine yeast. It is very hard to stabilize a wine that has residual wine yeast still floating throughout the wine.

Potassium sorbate is what has to be used to stabilize a wine when sugar is being added. While either potassium metabisulfite or Campden tablets should be used as well, each is not sufficient enough on its own to stabilize the wine.

Potassium sorbate stabilizes a wine in an entirely different way than any type of sulfite. It does so by putting a restrictive coating on the outside surface of each of the few remaining yeast cells. This does not necessarily kill the yeast. They will die on their own in hours or days. But it makes them unable to reproduce themselves. The ability to reproduce is the real threat that can manifests itself as full-blown fermentation.

If your wine is still even slightly visually cloudy, there may not be enough potassium sorbate to go around to do a complete stabilization. This is the downside to sweetening/stabilizing the wine sooner the necessary.