Hello Ankesh,
The EXIT_SAPLMV01_002 will be triggered for all the segments of MATMAS
Yes it is true that we cannot go back to MARC but internal table IDOC_DATA is already populated with MARC values when MVKE is being populated.
Therefore read table IDOC_DATA for MARC segment and modify it when you are filling MVKE segment,
Set a breakpoint for MVKE segment and check IDOC_DATA internal table content
Hope it is clear for you.
Thanks