You would have to go thru the process of re-opening your case, filing amended schedules, the whole 9 yards and it WILL cost you fees for re-opening and filing fees.
In general, it isn't at all necessary to re-open a Ch 7 to add a forgotten creditor. In most districts, if your case was a no-asset Ch 7, ALL pre-petition debts are considered discharged whether you listed them or not.