Adding New Relationships Between Standard Entity Types 
You can use this procedure to add new referencing relationships to the entity types delivered by SAP.
Prerequisites
A new relationship between two standard entity types can only have the type referencing. The relationship forms an attribute of the To-entity type and this attribute has the same name as the relationship.
The relationship uses the customer namespace YY* and ZZ*.
Procedure
With the exception of the settings for Enterprise Search – for which data model-specific structures are not generated – the enhancement procedure is the same as that for adding new attributes. For more information, see Adding New Attributes to Standard Entity Types.