When you first set up your iMessage account, you usually need to add a phone number and an Email address. Sometime later on you may need to add a phone number or e-mail. Maybe it is a work number or a second Email to contact you. This is not difficult to do, here is how that works.
iMessage New Number
In the Settings App on your iPhone, tap on your Name at the top of the first screen.
On the next screen, tap on “Name, Phone Numbers, Email”.
You will see this screen with phone numbers and e-mails (mine are blocked out, of course)
Tap on the “Edit” button to get to a place to add a phone number.
Just tap on “Add Email or Phone Number” which produces this confirmation drop down.
Then, just add the phone number or Email address, and it is added to your account. By the way, while you are in this area you can delete a phone number or Email as well.
Adding a phone number or Email address to your iMessages settings in iOS is easy. Once it is added, it will appear on all your devices.