The only way I have found to do this is to fix the field name in the Word
document that Outlook exports when you initiate your merge from Oulook
(assuming that's what you're doing. I wrote the following code a while ago
and you would probably have to create a toolbar button and run it manually
once Outlook has opened the merge main document.

For more background info you might be able to follow this link or search in
Google groups for, e.g. jamieson vba zip map fields scripting

http://groups-beta.google.com/group/microsoft.public.word.mailmerge.fields/brows
e_thread/thread/6a7be7708b7233f8/fde7b98f943986bb?q=jamieson+map+fields+vba&_don
e=%2Fgroups%3Fq%3Djamieson+map+fields+vba%26qt_s%3DSearch+Groups%26&_doneTitle=B
ack+to+Search&&d#fde7b98f943986bb

Sub ModifyZipFieldName()

' Macro by PJ Jamieson, March 2004
' This macro looks for a field name called "ZIP/Postal Code"
' in the header record of a mail merge data source and modifies
' the name so that Word automatically maps its ZIP/Postal code
' to the field.
' Written for use with Word 2003/Outlook 2003, but it may work
' in earlier versions
' Requires the "Microsoft Scripting Runtime" object library:
' You will need to use the VBA Editor menu command
' Tools|References to add a reference to this library

' This macro is intended to be used when a merge is initiated
' from Outlook. In that case, I believe the following assumptions
' can be made:
'   The merge data file is a Unicode format text file
'   (notice that by default it has a .doc extension)
'   Outlook reconnects to the data source and removes any
'   sort/filter options you may have defined

' The macro also assumes that it is OK to create/overwrite a
' file with the same name as the data source + ".tmp"

Dim oFileSystemObject As Scripting.FileSystemObject
Dim oSourceStream As Scripting.TextStream
Dim oDestStream As Scripting.TextStream
Dim sHeaderRecord As String
Dim sMergeDataSourceName As String
Dim vMergeType As WdMailMergeMainDocType
Dim vMergeDestination As WdMailMergeDestination
On Error GoTo finish

With ActiveDocument.MailMerge
If .MainDocumentType = wdNotAMergeDocument Then
  MsgBox "This document is not a mail merge main document"
Else
  ' Save merge type and destination
  vMergeType = .MainDocumentType
  vMergeDestination = .Destination
  sMergeDataSourceName = .DataSource.Name
  .MainDocumentType = wdNotAMergeDocument

  Set oFileSystemObject = CreateObject("Scripting.FileSystemObject")
  With oFileSystemObject
    ' This assumes the file is Unicode format (I think)
    Set oSourceStream = .OpenTextFile( _
                          FileName:=sMergeDataSourceName, _
                          IOMode:=ForReading, _
                          Create:=False, _
                          Format:=TristateTrue)

    ' Verify that the required string exists before going any further
    sHeaderRecord = oSourceStream.ReadLine
    If InStr(1, sHeaderRecord, "ZIP/Postal Code") = 0 Then
      MsgBox "Could not find the field name 'ZIP/Postal Code'" & _
             "in the merge data source." & _
             vbCrLf & "You will need to match fields manually"
      oSourceStream.Close
      Set oSourceStream = Nothing
    Else
      ' create the output file
      Set oDestStream = .CreateTextFile( _
                        FileName:=sMergeDataSourceName + ".tmp", _
                        overwrite:=True, _
                        unicode:=True)

      ' Make the substitution. The field names "ZIP" and "Postcode"
      ' seem to be recognised automatically by Word

      oDestStream.WriteLine _
        Text:=Replace(sHeaderRecord, "ZIP/Postal Code", "ZIP")

      ' copy the rest of the file
      Do Until oSourceStream.AtEndOfStream
        oDestStream.WriteLine Text:=oSourceStream.ReadLine
      Loop

      ' Close everything and replace the old file by the new one
      oDestStream.Close
      Set oDestStream = Nothing
      oSourceStream.Close
      Set oSourceStream = Nothing
      oFileSystemObject.DeleteFile _
        filespec:=sMergeDataSourceName, _
        force:=True
      oFileSystemObject.MoveFile _
        Source:=sMergeDataSourceName + ".tmp", _
        Destination:=sMergeDataSourceName
    End If
  End With
  Set oFileSystemObject = Nothing
End If

' Set up the mail merge data source etc. again
' You may find that you need to save and restore
' other settings
.OpenDataSource Name:=sMergeDataSourceName
.MainDocumentType = vMergeType
.Destination = vMergeDestination

End With

Exit Sub
finish:
Close #1
MsgBox "Error " & Err.Number & _
       " when trying to modify the ZIP field name: " & _
       vbCrLf & Err.Description
Err.Clear
On Error Resume Next
Set oSourceStream = Nothing
Set oDestStream = Nothing
Set oFileSystemObject = Nothing
End Sub
Peter Jamieson