What I did, In the first load, I kept the SOURCE_SYSTEM mapping in the transformation file with small adjustment in ID mapping, because I skipped the records prefixed by GR_. But in second load, I removed the SOURCE_SYSTEM mapping from transformation file so that it cannot impact the manually entered property value of SOURCE_SYSTEM and in ID mapping, skipped GLs starting from 1s & 2s. This is the only easy and quick work-around, I could find.
Got the test routine written from someone and tested, yes it worked.
Considering the best possible answer, which u suggested"ROUTINE", Closing the thread.