You may be able to do it by manually adding the tags that identify it as a 'podcast' with a third party tool. I had the opposite issue (trying to make MC NOT see files that were downloaded from an RSS feed as a 'podcast' anymore). The only way I was to fix it was to: remove the files from the library, shut down MC, move the files, manually delete the 'podcast' related tags using a third party tool, and then re-importing them in to MC.