1) Going to your Mail Options.
2) Selecting “Accounts”. If you’re in the All New Y! Mail, you’ll get bounced back to the old Classic configuration pages.
3) There should be a section about an “Extra Mail Address”, unless you already have a secondary account set up. With the non-Plus setup, you’re only allowed to have one mail alias, I believe. Provided that you don’t have another mail account (which you would see in the “Mail Accounts” section), then you can add on an alias from within the @yahoo domain.

As far as I’m aware, there isn’t any official way to clear up an account and then retrieve it immediately — I was generally more imagining that I’d delete my original account and try to re-register it again every few days until it worked. :D But you very may well be right that addresses take a long time to get freed up, in which case I apologize about the misinformation. Even though I work at Yahoo!, as far as Y! Mail goes, I’m pretty much just another user (with a tiny, tiny bit of inside information and the ability to poke around in their bug queue :P).

Regarding the general idea about merging accounts, Ryan K commented on his blog post that there was not any way to do so at the moment (because they want to make sure new users can get new @ymail or @rocketmail addresses versus existing users clogging up the namespace), but they’re thinking about whether they’ll add it as an option, so we’ll see.

For right now, though, yeah, I’m just squatting new accounts on my desired @ymail and @rocketmail accounts, in the hope that we’ll eventually be able to merge these guys together and I can do a quick “delete account / retrieve email address” swap.