Can I send as someone else using Mail.app on iPad?
Mail.app on iPad does not support sending as or send on behalf of. So there is no way you can do this via Mail.app.

But you can try this on a Macbook for example:
in Mail > Preferences > Accounts it will show the account you have set up. Next tothe emailaddress, type a comma and then the other email you want to send from. Save it and now when you compose a mail, you can choose which one to send from.