How to set up an alternate iMessage email address. From an iOS device:
1. Select the device you wish to change.
2. Go to Settings > Messages > Send & Receive > add another email
3. type the email you want to add.
4. Wait for a verification email from Apple. In the verification email you will need to follow the Link.
If you are wanting to change the email address for messages on your computer...
1. Launch Messages.
2. choose messages from the menu bar.
3. Select Preferences
4. select accounts