The problem is that you can't have two messages selected at the same time, so having Excel produce a macro which creates a message using Outlook from which you wish to call a data from another message, you are going to have to be able to identify the other message to the Excel macro. You would therefore need to get the Excel macro to first interrogate the selected message, then create the new message e.g. as follows, or better still just use the Excel code to create a reply to all to the selected message and replace the unwanted data with your new data.
Dim oOutlookApp As Object
Dim oItem As Object
Dim olMsg As Object
On Error Resume Next
'Get Outlook if it's running
Set oOutlookApp = GetObject(, "Outlook.Application")
'Outlook wasn't running
If Err <> 0 Then
MsgBox "Select the message in Outlook first!"
Exit Sub
End If
Set olMsg = oOutlookApp.ActiveExplorer.Selection.Item(1)
'No message selected
If Err <> 0 Then
MsgBox "Select the message in Outlook first!"
Exit Sub
End If
On Error GoTo 0
'Create a new mailitem
Set oItem = oOutlookApp.CreateItem(0)
With oItem
.To = olMsg.To 'If this is a received e-mail 'To' will be you?
.CC = olMsg.CC
'do stuff with your message
.display
End With