Changing signature and other email config settings.
Logging on the carrier's BIS page is not required ( at least not with Sprint) to change the default signature and other settings for your email configuration. It can be done from your BB without creating an account on BIS.
Have not test it but this may not work if you already have a BIS account.( I read somewhere something about it....)
Email Settings application will allow you to Add/Delete/Change your email settings including Signature and Auto BCC. It basically provide access to most if not all configurable settings for email.
Blackberry Curve 8330 Boost Mobile
You can modify the signature from the device. Go to email settings. Select the email account you want to modify. Easy!