ItemSend works when you send the Item from Outlook. It has already been sent by your application to the Outbox so by-passes the function.
If you don't have Outlook sending messages instantly, it should be possible to re-open each message in the Outbox using a macro add your CC to those requiring it and resend them. e.g.
Code:
Option Explicit
Sub AddCC()
Dim oFolder As Folder
Dim olItem As MailItem
Set oFolder = Session.GetDefaultFolder(olFolderOutbox)
For Each olItem In oFolder.Items
If InStr(1, olItem.To, "someone@somewhere.com", vbTextCompare) > 0 Then
olItem.CC = "someoneelse@somewhere.com"
olItem.Send
End If
Next olItem
lbl_Exit:
Set oFolder = Nothing
Set olItem = Nothing
Exit Sub
End Sub