Well if the base kit is 5 gallons to hit 6, you'd need to add a gallon of honey which is about 12lb.
Without knowing the sugar content of the honey it'd be hard to work out the gravity of the kit must with the added honey.
I would suggest that if you took 100mls of the kit must and then added 20mls of honey, mix it well then measure its gravity as its the same ratio of 5 parts juice/kit must to 1 part honey so you'd know the likely starting gravity.
What will increase the batch to a full bodied brew would be if you could get some grape skins to ferment with the batch. Your best bet would be if you could get the same variety of skins as the juice/concentrate in the kit......