You absolutely are able to do this. You may voluntarily pay any debt you wish after a bankruptcy, even if it has been discharged. You are under no obligation to do so, and the creditor can never legally request or force you to pay debts which have been discharged.
So, why would you want to pay a creditor if you don’t have to? Because all debts are discharged in bankruptcy, unless they are nondischargeable, like student loans, there is no obligation to pay. But if the debt is owed to a family member, or your family doctor, you may be inclined to pay this debt back. Not because you have to. But because you want to.
You should be careful, however, in choosing the debts that you wish to pay back after the debt has been discharged in a bankruptcy. Be especially careful if the debt is a credit card. Sophisticated creditors sometimes find ways to re-obligate you on the debt. I have a few clever agreements that create a brand new debt, one that includes an amount that was discharged, and the new debt becomes an obligation that you have to pay, even the part of it that used to be discharged. So, be careful what you sign, call your bankruptcy attorney if you don’t understand what will happen to the debt that was discharged.