Some wine makers step up the Alc% in this manner. Its not the best thing to do for beer. Why not add it in the beginning. It may be easier to take a hydrometer reading, or Brix reading to determine your Original gravity. So you will have a ball park figure of your alcohol content.
Its better to add corn sugar, or brown sugar,..etc at flame out for a higher alc% or use more malt in the boil. I would think When adding it to the secondary you might add a little bit of oxygen to your brew which is not the best practice, you also have a chance for contamination, and for long term storage this might not be ideal, but for a brew your going to drink early it might not be a big deal.
You wont kill your carbonation process, adding sugar to your secondary will give the yeast more to eat thats all and make more co2. Now if you had a big ol beer with an OG of 1.090 or 1.1, it could affect your carbonation, because thats allot of work for your yeast.....and when you go to bottle they might be to worn out to carbonate properly or it may take forever or it may not carbonate at all. My guess is if your going to add some sugar your not close to 1.090 or 1.1. ...On the big brews I like to use safale 56, I hydrate the yeast and add them to the bottling bucket then bottle. But I would not add yeast unless your starting gravity was 1.090 or 1.1 or it had been in the secondary for a long time and the beer was very clear....There are allot of factors. All you can do is try it.
