You are correct, if you do not have the "Add signatures to all outgoing messages", then you will have to manually add the desired signature to your message.

Unfortunately Outlook Express isn't a very robust program, so features like separate signatures for each account really aren't designed into it. The lack of features is probably one of the big reasons that it's no longer offered for newer versions of Windows. My suggestion would be to look into a more robust email program like Outlook or Thunderbird.

-SW