Welcome to the Ranch.
First step would be to find out if Yahoo provides some API which would let you create accounts "externally"
Without such an API, there is nothing you can do*.
*You
might try to monitor (using tools like
firebug) the calls made and the data/data format used when you manually create an account using the browser, and then use something like say URLConnection to do the same from your code, but this would not be a reliable solution.