SAP has come up with Substitute Authorizer option, wherein an Authorizer can substitute another SAP user to approve on his behalf for a specified time.
1. From Main Menu, choose Administration > Approv
2. The Substitute Authorizer for Approval Template window will appear.
3. In the Substitute Authorizer for Approval Templates window, in the Authorizer Code field, specify the user code of the authorizer for whom you want to define a substitute.
4. In the Substitute Authorizer field, specify the user you want to define as the substitute authorizer.
5. In the To Date and From Date fields, select the date range during which you want to the substitute to act as the approval process authorizer.
6. Click Update button.
This tip and other useful tips are available on SAP Community.