because my very first cider is still clearing, and the fact that ive never tried adding corn sugar or priming sugar to any other vessel than my bottling bucket, im not sure what the reaction to adding this sugar would be.
if youre just going by the 'bubbles' on the airlock, not a hydrometer, this is what i would assume:
im no expert but my best guess is this- just because there is no activity doesnt mean it has stopped fermenting, it has probably slowed to a point where you wont see much activity, thats just an assumption, but the hydrometer can tell you that for sure. thats just my two cents.
some of the other more experienced cider and apfelwein brewers may have a better idea of what to tell you if you posted the exact Recipe and Process.