I assume you are automating Outlook from Excel. This solution may work for you, but as you note it does rely on saving the attachment and re-attaching the manipulated version of the file. Assuming you can write the code which will "edit" the Workbook attachment, this should work for you.
Sub TestOutlookTemplate()
Dim MyOutlook As Outlook.Application
Dim MyMail As Outlook.MailItem
Dim att As Outlook.Attachment
Dim templatePath As String
Dim tempFileName As String
Dim attWorkbook As Workbook
templatePath = "C:\users\david_zemens\desktop\Untitled.oft"
tempFileName = "C:\users\david_zemens\desktop\tempexcelfile.xlsx"
Set MyOutlook = CreateObject("Outlook.Application")
Set MyMail = MyOutlook.CreateItemFromTemplate(templatePath)
MyMail.Display
For Each att In MyMail.Attachments
If att.DisplayName Like "*.xls*" Then
att.SaveAsFile tempFileName
'Now that you have saved the file, delete the attachment
att.Delete
'Open the file
Set attWorkbook = Workbooks.Open(tempFileName)
'Perform manipulation on the file
attWorkbook.Sheets(1).Name = "Sheet ONE"
'Save fhe file
attWorkbook.Save
'Close the file
attWorkbook.Close
MyMail.Attachments.Add tempFileName
End If
Next
'Send your mail (make sure you have added a recipient
MyMail.Send
Set attWorkbook = Nothing
Set MyMail = Nothing
Set MyOutlook = Nothing
End Sub