Check the schedule that listed all your debts to be considered for discharge. I thought I forgot to list one and discovered it was listed. If it was not listed, from what I understand you can't add it as your bankruptcy was already discharged. You may want to consult your lawyer.
If you were a no-asset case, even a debt inadvertently omitted from your schedule is discharged.
Even if you weren't a no asset case, the debt can be discharged for a period of time, but you would have to re-open the case to add the new creditor. I'd just show the creditor your discharge, and tell them it'll be cheaper for you to re-open than pay them, but you'll settle with them for a nominal (<$100) amount on the condition they remove all reporting to the credit bureaus that are derogatory or hurtful to your credit, and show the account as "paid in full/never late", or remove it entirely from your CR.