If a workflow has been configured to allow Ad-Hoc Approvers, Administrators can add approval steps while the workflow is running. 

Administrators can visit the Action Log and select Approvals to find all in progress workflows.

Select the Workflow that is Pending from the list.

In the Workflow Task screen, select the Add Approval Step button.

If adding the approval step as a replacement for the current step, drag the approver to the step that is currently in progress, and select Save and Update. 

Otherwise, add the Approval Step and click the Approve button, to approve on behalf of the current step and also add the ad-hoc approver.

Related Articles:

1. Administrator – Creating Workflow Approval Groups
2. Administrator – Configuring Workflow Field Definitions
3. Administrator - Workflow Associations
4. Administrator – Adding Users as Approval Steps in Workflows
5. Administrator - Configuring Ad-Hoc Approvers
6. Contributor - Adding an Ad-Hoc Approver Before Starting a Workflow
7. Contributor - Adding an Ad-Hoc Approver During a Running Workflow
8. Administrator – Adding Meeting Types as Approval Steps in Workflows
9. Administrator – Editing/Deleting a Workflow
10. Administrator – Adding a Late Exception Workflow
11. Administrator - Terminating a Running Workflow