Hi ti,
Present your data flow doing the same. its adding only new records.
First load you 100 records
2nd load you got 50 new records, those are added to cube.
3rd load you got 0 new record, so only 0 records added to dso and cube.150 are old records which you loaded from 2nd load.
When the coming record changed/new one, those only will add to dso and cube.
Have you got any data validation issue from users. as my guess may not.
Overwritten functionality works on the object which you kept under key fields area at dso design.
You may need to play with keyfield and data fields..
Thanks