You can still add sugar, people do feed in multiple steps along the fermentation. From what I understand it is supposed to keep the yeasties from stressing too much with a high starting gravity.
So, that said, if you want to add additional fermentable sugars in there now, as long as you are careful with sanitation, you should be ok. My suggestion would be a simple syrup or some thawed apple concentrate. Just leave it in the can, sanitize the outside of the can and then pour it in. Sanitize a piece of aluminum foil to cover the top while you are prepping everything and re-sanitizing the airlock. You'll need to swirl the carboy a little to make sure it's mixed in, but be careful not to introduce oxygen at this point.
But you really need those gravity readings to find out how much you started with, how much you are adding and finals.
Just my opinion of course