Adding SG readings?

So I have heard things about boosting different styles of brew with more sugar and/or yeast when racking to secondary. I have a cider going and I was thinking of doing just that (adding a bit more fermentable sugar). I used champagne yeast and had an OG of 1.062 and today it reads 1.000. If I do add more fermentables, how will I calculate abv%? Do I just take the new OG and FG, calculate abv% and then add the results to my current abv%?

This is a bad idea. Metabolic by products of yeast fermentation yield undesirable compounds that should be aged out over the course of at least two weeks. Restarting fermentation would reset the clock so to speak.