You can do this from Outlook by opening an Outlook Application Object, setting up your context (which is something that has been posted in the forum a few times), creating the mail message object, and then opening something called an Inspector.
At this point, you are in the selected Outlook mail editor (these days, usually Word) and can do what you need. You can manually update the To:, CC:, and BCC: lists, change the subject, and attach objects. You can set importance, set encrpytion (if that's what you needed), and do a few other useful things.
When you are finished with all the required gyrations, THEN you can do the .Send operation. However, once you send it, depending on your Outlook settings, it moves to the Sent Items folder within your .PST file and at that point, editing is sort of like closing the barn door after the horses have escaped.
I don't think I've ever used the raw .SendObject because I've always needed to give the users a way to update the messages they are sending, so I have always used the Outlook App Object methods.
If you were using CDO (Collaboration Data Objects), you could send messages via SMTP but I don't know that you can so easily edit that message before sending it. There, I think the moment you activate CDO with a message having the required parts, that message is gone.