Value mapping in the ERP system during IDOC creation

Question

We have a PI File-IDOC scenario.
We can translate simple mappings such as currency or unit of measure in PI(7.1) using the Value Mapping function. For complicated mappings involving business logic (e.g. Tax code which is derived from multiple fields) we want to perform these mapping in the ERP (ECC6) system. Is there an approved generic SAP standard process where these value mappings can be done in the IDOC creation which is still valid when re-processing?
e.g. a specific BADI or enhancement point recommended for these mapping?
There might also be a requirement to add segments based on value mapping logic for example when a tax record segment is only required for a non-zero tax code which is mapped via business logic.
Can someone please direct me in the right direction.

Answer 1

Thanks for your answers but I assumed that using a user exit / Enhancement point was obvious. What I'd like to know is a generic entry point that I can estabolish a IDOC enhancement framework. Somewhere I can call a class containing methods linked to IDOC message types
e.g. Call similar to my prototype (this will be where dependant on mappings certain segments will need to be inserted such as tax segments.
  ASSIGN control-mestyp TO <mestyp>.
    CALL METHOD (<mestyp>)
      EXPORTING
        control        = control
        data           = data
      IMPORTING
        have_to_change = have_to_change
        protocol       = protocol
        new_entries    = new_entries.
The "entry point" must be processed both at creation and reprocess. We are also dealing with Inbound IDOCs not outbound