I don't think you can. Thunderbird only allows "from:" addresses to be selected from those installed as incoming accounts (i.e. using POP or IMAP). I believe you can now edit the from: address in an ad-hoc manner, but it seems to be on a one-at-a-time basis and wouldn't suit repetitive usage. The nearest I can think of is to set up a bogus account as a second identity on an existing account, but then of course with many email providers, you might have trouble using an unknown address as the sender. You can set up your "sending" account as normal and just disable the automated fetching of messages, but this wouldn't prevent a user manually doing a fetch.