People Import using the excel file from the app Alert email issues.

People Import using the excel file from the app.   When I imported about 20K users it is supposed to write over the fields that have been updated on the account.  However, one column was misaligned by a row and updated the account with the wrong information on the Alert Notification field. When Looking at import file after the correction import I noticed there is no Alert Field in the form.  There is Alternate but noting to update the Alert notification email. field. When importing with the correct columns now I cannot update that field.  That makes for a huge problem if different student are being notified.  Please help.