Table sugar will add 45 points per gallon per pound, so you have to take that figure and calculate your addition based on wort density. Papazian has a calculation for it, I think.
Or you could sit down with some brewing software, get your numbers to where you're at your current gravity, then slowly add sugar in amounts to the recipe until it hits your target gravity, then add that amount of sugar.
I think the math on this is pretty tricky, so I'd probably do it trial and error with the software.
