I don't understand the workings behind the signing process but, to me, it looks like you can write a program that can take any address and your own message to create a signature so that you can pretend it's your address.
You can only sign with a sending address if the private key for that address is in your wallet. You would need to break the private key cryptography central to bitcoin in order to do what you describe. If you can do that, there's $1 billion available for the taking, so get to it.