hi edi,
you can add required field to Extract structure directly at SE11, give ext str name and click on append, give some name ZZext str, add fields.save, back,
appende fields will be added to yout ext str as last row.
go to RSA6, choose your data source, added fields was hidden, un hide it. save and back.
Go to CMOD, choose existing project, RSAP00001, Exit - EXIT_SAPLRSAP_001, INLUDE ZXRSAU01, double click on it, its take you to SE38 or Directly go to SE38 - INCLUDE ZXRSAU01, edit mode and maintain logic with help iof abap.save, syntax check and activate.
All changes capure in Transport request.
Do the test case at RSA3. if you field and data. then replicate data source into bw side and activate. do the rest of steps.
All changes move to prod once you got design as you needs.
Thanks