The only way to do that is to insert the necessary number of blank records
at the beginning of the data source.  If your data source is a Word
document, the following macro will do that for you:

'Macro created by Doug Robbins 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

The macro needs to be run when you have the mailmerge main document already
set up and it will send the merge to a new document that you can then print
out onto your labels.

If your data source is not a Word document, you can create a Word document
as the data source by creating a Catalog or Directory type mail merge that
uses your original data source in the main document of which, you insert a
one row table with as many columns as there are fields in your data source
and then you insert the merge fields into the cells of that table.  Then you
execute that merge to a new document and then insert a new row at the top of
the table that will be in that document and into the cells of that new row,
you enter the names of the merge fields.  Then save that document and use it
as the data source.