If you want to copy the standard workflow of the leave request approval process (WS12300111) and are adding antoher approval step (or you just want to add an esacalation where you set the approve workitem to obsolete and create a new approval step for the next approver) you have to implement the following BAdI. In detail you only have to add the new step ID in the filter, else the approver will not see any approval or reject buttons in his inbox. See details here.
As second step you have to add the Worklow in the customizing. You’ll find further information here.