Change derived data to stored

From my experience, you have 2 options: either rectify the problem in Staging or the Cube.

Staging: this will depend on the source data. If that imports the Cube dimension members (ie all the transformation rules are * to *), then you could potentially write a derivative rule that creates a record with amount = 0 for all the intersections that are in the previous period but not the current. If the dimension members need transforming, then it is more tricky as you need to reference transformed data and create a new target data record.

Cube: have a calculation that checks the derived status (api.Data.GetDataCell("POV").CellStatus.IsDerivedData) and sets the value to be 0 if necessary. You can restrict the check to Origin Import.

Staging I think is probably the way to go if the source data does not need transforming. If that is not the case I think Cube is probably safer and certainly easier.