I don't think you can...I think that the whole point was that it was Apple's way of rewarding iOS device users with a free way to message each other whilst also attracting new customers to the genre.
You shouldn't have to register phones in order to be able to send messages to them, just send them a message. I can send to non-iOS 'phones using iMessage on my iPhone, however, I have to pay for the message.
It's not a matter of registering your phone. You need to have an iPhone, iPod, iPad running ios5 or 6, or a computer running mountain lion operating system. No third party hardware or software can use iMessage. It is a great function. Very useful for me when in Australia earlier this year.
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