In the "Accounts and Import" settings, you add an account and instead of choosing "Send through GMail (easier to set up)", choose "Send through <example.com>'s SMTP servers".
And then you need to enter the username, server, password, etc for the other account, which can even be another GMail account.
And then you need to enter the username, server, password, etc for the other account, which can even be another GMail account.