Unfortunately, adding a new view to the web client isnt as easy as mucking with the skin. There's a lot of javascript behind each view that drives the UI.
We're slowly getting to a point where the rendering code is being separated from the program logic using templates. So eventually, you will be able to modify a template and override an existing one. But we're not completely there yet. And even then, is some cases, you'll have to do a little javascript kung fu.
In the meantime, there's a couple of existing enhancements that you may want to vote for depending on whether either solves you're what you're trying to do:
http://bugzilla.zimbra.com/show_bug.cgi?id=450 http://bugzilla.zimbra.com/show_bug.cgi?id=14863