Because YACCS (and other third-party commenting services) technically aren't a part of your blog, I don't know that they can be imported. At least not smoothly. You're basically exporting two separate sets of data: The Blogger content, and then the YACCS content. There would have to be an identifying tag for each Blogger post that matches with a tag in each YACCS comment box. I don't think Blogger makes allowances for that.
I know HaloScan allows you to export/backup your comments, but I don't know how that syncs with blog content.