Hi Csaba,
It would have been a good idea to mention the 3rd party Oracle databases in your original discussion. It is quite crucial information since the SAP ECC Business Content DataSources 2LIS_03_BF, 2LIS_03_BX and 2LIS_03_UM cannot be used in your situation.
Anyway, analyzing the Business Content will give you an understanding about non-cumulative key figures and their application in the context of Inventory Management. A part of it might even be reusable.
As already mentioned, please also refer to document How to Handle Inventory Management Scenarios in BW (NW2004) and SAP Note 1548125 - Interesting facts about Inventory Cubes for more information.
Best regards,
Sander