After a thorough searching and talking to some gurus, there is a way to load the master data with ECATTS that are included into SAP Best Practices' building blocks.
1276914 - Advanced Installation Options of SAP Best Practices Packages
But the best way for us was to reuse one's LSMW project for this purpose. LSMW is an old but a still good tool for data migrations, if you control the technical part of SAP well of course.