Maybe something like

Sub AddBCC()
Dim olFolder As Outlook.Folder
Dim olItem As Outlook.MailItem
Dim strBCC As String
    On Error Resume Next
    strBCC = "someone@somewhere.com"
    Set olFolder = Session.GetDefaultFolder(olFolderDrafts)
    For Each olItem In ActiveExplorer.Selection
        If olItem.Parent = olFolder And TypeName(olItem) = "MailItem" Then
            olItem.Recipients.Add(strBCC).Type = 3
            olItem.Save
        End If
    Next olItem
lbl_Exit:
    Set olFolder = Nothing
    Set olItem = Nothing
    Exit Sub
End Sub