The way to do this is to insert the necessary number of blank records at the
top of the data source.

If the data source is a Word table, the following macro run after you attach
the data source and set up the mail merge main document, will ask you how
many blank records should be added and will temporarilty add them to the
data source and then execute the merge.

If your data source is not a Word document, you can use a catalog (or in XP)
a directory type mailmerge to create a Word document to use as the data
source.

Macro to set the first label on a part sheet of labels for a label type
mailmerge.
Dim MMMDoc As Document
Dim dsource As Document
Dim dtable As Table
Dim i As Long, j As Long
Set MMMDoc = ActiveDocument
With MMMDoc.MailMerge
   If .MainDocumentType = wdMailingLabels Then
       If .State = wdMainAndDataSource Then
           Set dsource = Documents.Open(.DataSource.Name)
           Set dtable = dsource.Tables(1)
           i = InputBox("Enter the number of labels that have already been
used on the sheet.", "Set Starting Label")
           If IsNumeric(i) Then
               With dtable
                   For j = 1 To i
                       .Rows.Add BeforeRow:=.Rows(2)
                   Next j
               End With
           End If
           .Destination = wdSendToNewDocument
           .Execute
       End If
   End If
End With
dsource.Close wdDoNotSaveChanges