Migrate CSV source serial
Meanwhile I got this figured out. The second argument of MigrateSQLMap is a field => schema array and you can fill in those fields in function prepareKey($source_key, $row) as necessary, say

function prepareKey($source_key, $row) {
  $row-&gt;vendor_id = $this-&gt;vendorId;
  return parent::prepareKey($source_key, $row);
}


Note: prepareRow is too late, you want prepareKey.