How to extend combined datasource

Hi

I have extended Project BO with an field, the same field I want to use in a report which is based on combined Datasource "Project, Baseline and Snapshots".

From SDK I can enhanced only Project data source, I thought this field will reflect in Combined datasource but this extension field is not coming in combined data source.

Any help how to extend combined datasource from SDK.

Thank

Sunil Maurya