Adding that much sugar to secondary (after you've removed most of the yeast) sounds like a phenomenally bad idea to me. A great way to make bottle bombs too.
If you're going to do this - brown sugar is just white sugar with molasses. Skip the brown and just add lactose and molasses. I'd also take a few small glasses and dose samples at different levels to see how they taste first.
Agreed. If you're bent on making it sweeter by addition of molasses and don't want bottle bombs (assuming you're bottling), it may be a good idea to pasteurize the bottles once you get to a carbonation level you like.
Personally though, I would just leave it alone. You'd be amazed at how different some beers taste from fermenter to bottle.