Within Teamwork.com, you can change the creator of an individual task.
Go to the List area of your project and select the relevant task to open the task details view.
When viewing the individual task, click the options button (three dots) in the top right and select Modify Properties from the dropdown menu.
In the modify modal, you can select a new creator from the dropdown. You can pick any user who has permissions to create tasks on the project.
Click Apply Changes to update the creator of the task.
For more information, see: Changing Completed Task Details