I think the usual scenario would be you will get a mapping document from BAs, and then you will use the ETL tool to create that mapping.
I don't know the technical work, but one real time scenario I worked on is users will enter data in front end application. Whatever user entered in one session will be stored as a xml file. ETL guy would process that xml file into staging database and then into the normalized data warehouse. From there there would be another ETL process to send the data into OLAP(denormalized) data warehouse for reporting.