Short answer - not with the current News (RSS Newsfeeds) module.  The only thing this module is really doing is taking your URL for the newsource, generating a web request, taking the response, and formatting it with the specified XSL transformation file.

Basically, every news feed differs in it's use of the pubDate element, shouldn't but there it is.  The specs call for a particular format for the date, but XSL really doesn't know anything about dates, so it's just treated as text.  Scripting in XSL is possible, so theoretically, you could parse the element, change it through code, and sort it, but scripting in XSL gives bad performance issues, as well as, lends itself to some cross-site scripting security issues.  In general, posts in feeds are in reverse date order, meaning date descending order, you could check for the pubDate element, and do some sorting with it, but as I said, every feed is a bit different, so you would need to eyeball your feed and try coming up with a suitable XSL to match your specific needs.