Before you can start using an email address for iMessage, however, you'll need to make sure it's not an existing Apple ID.
Note: If your email addresses are @icloud.com, @me.com, or @mac.com your email address is automatically an Apple ID. If you have an iTunes account, the email address associated with that, and with FaceTime, are also automatically Apple IDs.
You can only have one Apple ID linked to iMessage account at a time.
Here is how to add non-Apple ID email addresses to iMeassages:
- Launch the Settings app on your iPhone or iPad.
- Scroll down and tap on Messages.
- Now tap on Send & Receive.
- Select Add Another Email... underneath the You can be reached by iMessage at: section.
- Type in the email you'd like to add and hit return on your keyboard.
That's it. You'll receive an email asking you to confirm you have access to the entered email account. Once you do that you're ready to start sending iMessages from the newly added address!
Hope this can help you.