Template for those Tables are located :
SAP\Data Transfer Workbench\Templates\Templates\Inventory\Price Lists\Special Prices\Discount Groups
OEDG - EnhancedDiscountGroup.xls
EDG1 - DiscountGroupLineCollection.xls
AFAIK, you can only Add/Update via DTW.
So to do the maintenance of Discount Group, you may need to use SDK to Add/Update/Delete.
Going through Excel Macro has done the trick for me.