I’m fairly certain you can create an account in iOS mail with whatever address you want, leave the incoming credentials blank, and choose an existing SMTP account for outgoing. At least that’s how I used to do it.
Just tested it. I can't save the new account in the Mail settings when I leave the incoming credentials blank :-/ It works when I add the real IMAP account though. And the Mail app seems to be clever enough to not duplicate everything in the combined Inbox view. Thanks, at least that's a tradeoff.