Here is a macro solution:

Sub ScratchMacro()
'A basic Word macro coded by Greg Maxey
Dim oPar As Paragraph
Dim oRng As Word.Range
  For Each oPar In ActiveDocument.Range.Paragraphs
    If oPar.Style = "Heading 1" Then
      Set oRng = oPar.Range
      oRng.InsertBefore ". "
      oRng.Collapse wdCollapseStart
      oRng.Fields.Add oRng, wdFieldSequence, "Nums"
    End If
  Next
lbl_Exit:
  Exit Sub
End Sub
However, I would select one of the Heading 1 paragraphs and the use the Home>Paragraph>Multilevel List control to set a predefined or create a custom numbering style.