This is behavior of Thunderbird is arguably poorly implemented, but you can add any account with any settings without contacting a server to verify those settings.
TL;DR -- After initial account verification failure, manually enter your email server settings (including any bogus ones) and press "Advanced config" when it appears. Then press "OK" once the account
Server Settings are displayed. Your account is created.
First add an account via the normal process:
Wait for Thunderbird to fail the settings or simply click "Manual config". Either of these options should bring you to a screen to enter account information manually. Enter any information you wish (use accurate information for the IMAP settings), but all fields should be filled out (i.e. make up an SMTP server and don't forget all the authentication stuff):
Press "Advanced config" (you don't need to bother with "Re-test"
/"Done"/"Cancel") and the
Account Settings screen should come up with
Server Settings highlighted for the new account (which has miraculously been added without a single acknowledgement that this has occurred):
No fancy stuff, just press "OK" and you should now have a new account available in the main Thunderbird pane (at the bottom).