There really isn't any batch editing of the content type stuff. What you could do though is, if there are currently font tags in the posts you will import, you could open the export file in a plain text editor (after making a backup) and then do a find and replace.
If you are talking about adding formatting where there wasn't anything to begin with, then that would be more difficult to do.
The other option would be to set the new blog to private, import it into the blog and then get the Windows Live Writer blog client (free download) and it will connect to the blog, download all the posts and then you can work on them in WLW and it will update the posts on the blog. That might be easier than doing them all in the wordpress editor, and WLW has a lot more features and capabilities than the wordpress editor.
That is of course if you are on Windows. If you are on Mac, there is Ecto, but it isn't free like WLW. It is inexpensive though at around $20.