Hi Jens,
There is a way to create the obsolete 3.x DataSources. The default setting hides all obsolete functions for which a newer technology is available.
You can deactivate the default setting in Customizing:
SAP NetWeaver Business Warehouse > General Settings > Show obsolete Functions of Data Warehousing Workbench
T/code: RSCUSTV32
As an alternative for ETL using the data staging BAPI, you could evaluate Data Services (BODS). It can work with 7.0 DataSources and is a more future-proof approach.
Best regards,
Sander