@wsmwk I don't believe that's what the OP means. There is not an option to include signatures in the Account Settings menu nor is there an option to include the signature from the Insert menu when you're composing an email.
I think ideally, it should be an option to flag it for all replies AND to include another option in the Insert menu if you decide that a specific email requires the addition of your signature. Right now, I have to go in to the Account Setting menu and copy and paste it into any emails I want to reply to or forward on.