Hi Lewis,
1. You can add an extension field using Adapt Mode.
To add an extension field -> you have to first go to the purchase order screen where you want to create an extension field. Then using Adapt option -> Enter Adaptation Mode -> select an extension field and using Further Usage , add this extension field to the data source available under Datasources and Reports tab. You have to check whether this datasource is available or not under this tab.
2.I also tried to create a new report using datasource :GR/IR Inventories- Balance Summary and could able to see all Key Figures.
Can you please let me know how you are creating a report here?
Regards,
Kopal