I want to add an additional field to the approval workflow form so that approver can also fill the values for that field. The field exists in Tasks list.
You can make changes in Task Form. Basically approval form is Task form where approver can approve the task. If you are developing Workflow by Visual Studio, then given link can help you. https://prasadgavande.wordpress.com/2015/04/06/change-task-edit-form-of-sequential-workflow-in-sharepoint-2010-using-visual-studio/