I had my bankruptcy discharged in 2009 and now I found out that I forgotten a creditor. Is there any way to add them? I got a letter yesterday saying to appear in court because I'm being sued by them.

Where do you live and what type of bankruptcy did you file? In some states, with a Chapter 7 "no asset" case, an omitted creditor is not prejudiced because they would not have been paid anything anyway even if you did name them - so, it would be discharged now. Perhaps you could also reopen the bankruptcy.

Once it is discharged, no you can not go back and add people.

If it is merely filed and not discharged you may request to amend your filing.

In a Canadian bankruptcy, all debts owing at the date of bankruptcy are provable and subject to discharge in that bankruptcy regardless if they are listed in the bankruptcy documents EXCEPT for any dividend that the creditor would have received had they been included in the bankruptcy.

Example:
Total Debt: $10000.00 (including missed debt)
Missed Debt: $1000.00
Total Dividend available to Creditors: $500.00 after fees
Bankrupt would have to pay $50.00 (10%) to missed creditor