Yahoo! currently only puts apps in the gallery written by developers it has partnered with.
We don't yet have a way to share apps with other users that has a lower barrier-to-entry.
If you have developed an app that you think will make mail better, post the appid here and if we like it, we'll be in contact with you.
I found very good doc about how to do an application, but What is the process of having your application listed in Gallery?
thanks,