Unable to add another email in iMessage or login with a different iCloud account on iOS 11
To use another iCloud account, please go to

Settings > Messages > Send &amp; Receive and tap 'Apple ID: >your Apple ID&lt;' > Sign Out.

You also need to sign out of iCloud in Settings > iCloud in order to use another Apple ID for messages/iMessage.

Now you can sign in with another Apple ID.

To add another email address you be reached with using iMessage, manage your Apple ID in a browser and add another email address there.