BW Extract function module

I have a problem - I am trying to create a generic extract function module. The problem is that the template RSAX_BIW_GET_DATA_SIMPLE uses one database select. However for my purpose I need to select from various tables in order to build up my internal table.

It is for PO Service orders so I have to read headers then detailed lines then other tables to get account assignments etc. How do I transfer this to E_T_Data using the required logic for the fetch and open curser etc.