Your latest comment makes your requirement a bit more clear.
How about this:
MapTableB:
MAPPING LOAD ID, false() as Flag RESIDENT TableB;
TableA:
LOAD * FROM TableA-Source (options)
WHERE ApplyMap('MapTableB', ID, true());
This will load all rows from the TableA data source except those presejnt in TableB (without loading TableB). If TableA is already present in your data model, transfer it to a new RESIDENT table using this technique, drop the first TableA and rename the second table to TableA.
