May I ask why do you want to create multiple accounts on Yahoo? If you'll be using it to spam a website, better not consider it. IP Address can be detected unless you'll go somewhere to register your new Yahoo messenger account. Jut my opinion.
Yep you can make it by using yahoo alias you find this option in your mail account settings i guesss and its not a whole sperate account but you can make diff id's and use it for example: aamir123 at yahoo.com aamir1234 at yahoo.com both runs on same account