Short answer, I don't think so.

The only ListView available on the entire list is the Subject view. If
you click on a specific Discussion Topic you will see a Flat list of the
posts to that topic. If you want to indent these posts based upon their
position within the thread, you can choose the Threaded view. But that
is as close as it gets.

I think that one of the Community Kit for SharePoint (CKS) projects has
an enhanced Discussion Board. Check out:
http://www.codeplex.com/CKS

Also, you could potentially create a new view in the browser, open that
page within SharePoint Designer, and create an Expand/Collapse
functionality by converting the List View Web Part to a Data View Web
Part; but I haven't done it so I can't say for sure.