At the moment, you can't. We do plan to open up something similar soon.
We plan to implement Yahoo!'s webcam protocol in the future.
Implementing voice chat for Yahoo! Messenger 6.0 and below is a bit of a mystery. All we know is that it used RTP with its own extensions, but we do not know how those extensions were set up. However, voice chat for Yahoo! Messenger with Voice and above (7+) might be do-able, since we believe it used SIP, but we don't know if this was P2P or centralized.
As for Yahoo! Chat's voice features, if we implement Yahoo! Chat at all, that consistently used the RTP-based voice servers used for classic Yahoo! Messenger clients right through the last version of the client. We don't plan to implement it for Yahoo! Chat any time soon either.
Because of old limitations, we're required to do that.
We're hoping to move away from these limitations soon, but you don't need to worry about it.
Not in the near future, they're not really an important feature anyway.
It's easy, just send your username to your friends! (If you want a 4-character username, you should consider donating).