Email directly to iMessage?
There is no way. iMessage is a proprietary service, and the sender needs access to the receiver's public key to send a message. Techcrunch explains the whitepaper Apple released on iMessage's implementation in this way:


  When someone starts an iMessage conversation with you, they fetch your public key(s) from Apple’s servers. Before that message leaves the sender’s device, it’s encrypted into something that only your device knows how to decrypt.


As email doesn't have that handshake component, there's no way for you to encrypt the message client-side.