Hi Sandeep,
Is it custom workflow or its SAP standard workflow ? If its custom then hope your Approval determination logic is also done based on some custom code. Please put a small piece of code logic for your scenario. Go with below solution, it will never require any change in future:
1.) Check the position of the PERNR.
2.) If position is Director then populate Approval table with Assistant ID elseif its not Director then keep the current functionlity as it is.
Also If you are sending notification through workflow may be changes requre there also.
BR,
Prakash