Yes, we can modify the due date in a Review Task via Edit Properties, but we don't have a similar option for a customized workflow.
We have a custom workflow for approving PDF files. The author fills up the details in the form (see the attached screenshot), and when they want to modify the set due date, there's no option for them. When they select the workflow from Projects -Workflow tile (see attached screenshot), they have the option to "Terminate" the workflow, but no option to edit its properties.
Custom workflowTerminate option