Hi Carlos,
There are two options.
Option 1: You can enhance the DataSource with the field Material Group and write a small piece of coding to enrich the extracted records with the Material Group (MARA-MATKL).
Option 2: You can add the Material Group in the Transformation in BW. You have to make sure that proper loading sequence is guaranteed, i.e. first Material master data and after that your new data flow.
Best regards,
Sander