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.
I agree...don't do it.Let it age (as you should do with any RIS)...it won't gain in mouthfeel, but the aging will add some nice complexities to the flavor (which may in turn serve to make up at least somewhat for the lack of "thickness" you describe).
I'm not a supporter of trying to fix beer- especially once it's left the primary. If you feel like it needs some dry-hopping that is a different matter, but I can almost guarantee you won't be happy by throwing the kitchen-sink at this already huge RIS.Congratulate yourself on a successful fermentation and let it condition. Six months down the road you will be thanking us for trying to dissuade you from ruining your beer.Once it's carbonated you will get that mouthfeel back that you think you are missing.