After the data is migrated, perform an update on all records to set the original date in a standard Created Date field.

Create a new field on the Contact object to capture the Created Date. Hide the standard Created Date field using Field -Level Security.

Enable "Set Audit Fields" and assign the permission to the user loading the data for the duration of the migration.

Write an Apex trigger on the Contact object, before insert event to set the original value in a standard Created Date field.