Transformer's focus is on associating data sources and values in their columns with the levels and measures you define in your multidimensional model. Transformer processes each data source independently, defining categories and/or generating measure values for PowerCubes. For this reason, Transformer does not perform database-style joins between columns in your source data files.
If you require joins between database tables, use a query tool (such as Impromptu) to create the joins prior to adding data sources to the model.