How do i add someone else's iPhone contact number to messaging on an iPad?
According Apple Suport

Send a message

Open the Messages app and tap  Compose button.
Enter a phone number or email address, or tap  Plus button and choose a contact.
Type your message, then tap Send.

Forward a message

Open Messages and follow these steps:
Touch and hold the message bubble you’d like to forward. Then tap More.
Enter a contact.