iMessage with iCloud mail aliases?
In trying what I think you are wanting to do.
I had to enter my real email address and password to activate the service.

(Remember your real apple ID youname@mac.com/*yourname@me.com* is your user name and an alias email address is NOT)

Once I did this. My apple id is used for the first email address.
I can then add another email address below it.

the second one was one of my alias emails to my apple email.

Logging out of this service will remove both addresses.