Custom field data not populating in target from staging in standard data entity AX7

We have a requirement for adding some fields in standard data entity object in AX7. Data entity is "BankJournalLineEntity" and I have added a new field called "ExchRate" in staging table and data entity object as mapped field to LedgerJournalTrans ->ExchRate. Now data is populated in staging while execution but not in target tables. I couldn't find any method to override for updating data from staging to target. In some of the standard entities I saw a method called "copyCustomStagingToTarget" . Do I have to write similar logic? How can we achieve this? please help