I'm not sure how the Really Simple Podcasting plugin manages episodes, that plugin never got that many downloads so we never bothered to handle importing for it. If they store episodes the way WordPress wants (as a custom field "enclosure", then PowerPress will pick it up without problems. Otherwise, I would enable both PowerPress and really simple podcasting, edit the 6 blog posts in question, and copy/paste the media URL from the really simple podcasting plugin to the PowerPress media URL field. You should not have to reupload any media, as long as the URLs are copied over you're golden.
As far as your podcast feed is concerned, what is the current address of your podcast feed? I'm not familiar with that plugin, so if they used an abstract URL for the feed, that will be a new challenge.