Hello Ingo,
since SPS09 you can use declarations from other CDS files using the "using" option.
For details please check following:
- SAP HANA SPS 09: New Developer Features; Core Data Services (chapter "Multi-File Support")
- http://help.sap.com/hana/SAP_HANA_Core_Data_Services_CDS_Reference_en.pdf (chapter 2.2.2 External Artificats in CDS)
Regards,
Florian