Public Sub main()
  Dim oRange As Range
 Dim oComment As Comment
 Dim sText As String
 
 'Use object variable to hold range. In this example Sheet1 Cell A1
 Set oRange = Sheets("Sheet1").Range("A1")
 'Use object variable for comment
 Set oComment = oRange.Comment
 'text to be added to the comment box
 sText = "Hello"
 
 'If Sheet1 Cell A1 has a comment then append "Hello" to the end of the comment text
 If Not oComment Is Nothing Then
 sText = oComment.Text & sText
 oRange.Comment.Delete
 End If

 'Add a comment with the contents of sText
 oRange.AddComment sText
End Sub