Not sure if that's a difference for a kit; I've added sugar directly to must many times with fruit wine, the SG still rises and the yeast still eats it.
I agree on the carboy comment, though, this is always during primary for me. I've never added sugar after racking except to sweeten the wine. But then I rarely do a secondary fermentation, I usually ferment to dry in the bucket.
If you've already fermented, you can "cheat" by adding everclear to the wine, but it will dilute the flavor a bit (you COULD make an everclear f-pak, buy some grape concentrate that matches your wine, mix it with the everclear, and dump it in, however this will also sweeten your wine as wine concentrate has a lot of sugar).