Unfortunately there is no way to morph the message without creating a new message, which means the time stamp will always be different and you need to process the attachment.
Instead of using the exact code in the example for the Phone message creation, you could modify it so that it creates a Posted Phone Message rather than a group phone message. You could then include the date/time stamp details of the original message in the body text of the phone message. As you pointed out, this is not ideal because a) you would still need to transfer the WAV attachment and b) the creation date of the message does not match the original.
As an aside, just for future reference, see the new AddExistingMessage command at http://developer.novell.com/ndk/doc/gwobjapi/index.html?gwobjenu/data/h7ikbsqg.html . We have not used this call yet as it was only documented last week. In any case this new call will not solve your current requirements. It allows the creation of a new message without actually sending it (incoming, outgoing, draft, personal), although it appears that this will only work with an email message type.