Sure, there are a couple ways you could do that. The Discussions component in Alfresco Share provides an RSS Feed, which will give you an XML representation of the topics and comments.
Another way to approach it is to look at the individual content items for each topic via the Javascript API, REST API or CMIS. If you look at the site through the node browser, you'll see that each site has a cm:discussions container node of type cm:folder. The children of that node are the discussion topics, of type fm:topic. The children of the topics are the the comments, of type fm
ost.
You could write a simple Javascript web script to fetch each topic and the associated posts, or do it through out of the box web service calls, either one would work. The Javascript console would also be a good way to solve this if you only need to do it once. A little server side Javascript could get the site, get the discussions folder, iterate over each topic and its posts and write all that out to a new content node.
