Let me see if I understand. At some point you fell behind in child support and had to "catch up" payments. So you had the regular support plus some extra amount taken out of your check automatically. At some point, the balance owed was paid off but you didn't stop the "extra" payments. You still presumably owe child support for the child but want to add up the "extra" paid after the balance was paid off and use that to reduce the period of time that you have to continue to pay child support. The answer is "no". The amount ordered is a floor not a ceiling. Parents are expected to contribute more than the minimum payment toward their child's support, be that buying them clothes, paying for sports lessons, feeding them when they see them, taking them on vacations, etc. That "extra" was a gift. You are responsible for paying at least the minimum amount due until the child reaches the age in the order or your state deems you no longer financially responsible.
You can file in court to have the extra modified if the arrears has been paid off.
