Hi Salman,
Unfortunately in AX 2012 (and D3FO currently) it is not possible to send a completion notification to a user based on a managerial hierarchy. As you can see from this screen shot, the only providers that you can select from are the Participant (supports security roles and user groups), Workflow user (supports workflow owner and workflow originator), and User which supports a specific user. A developer could add a participant provider; however, this would be somewhat complex to implement. Another option from a development standpoint is to add a automated task that sends the email based on whoever approved the workflow. This too could be complex as there could be multiple approvers and so on.
