Can iTunes combine identical podcasts? Somewhat similar to this question, but I don't just want to import.  I have a bunch of This American Life podcasts that I downloaded using iTunes on another computer.  I also have a This American Life podcast subscription on my current Mac with a bunch of podcasts downloaded.  

I have already imported the old ones into iTunes.  They show up as two separate podcasts, with the same name and metadata.  The old ones have a "subscribe" link but I haven't clicked it, they're just sitting there.  The new ones are subscribed and regularly downloading new episodes.

How can I merge the old ones into the newer, subscribed version of the podcast in iTunes?
This has been bugging me for years, most specifically with This American Life and To The Best of Our Knowledge.

The tag that's causing them to be separated is not available via iTunes -- you need an extended tag editor. On the Mac, I used ID3 Editor. 

Instructions specifically for ID3 editor:

• Open a "good" podcast file (you can drag it straight from iTunes onto the ID3 Editor icon in the dock to open). 

• Click on the Podcast tab. Look for the field called Feed (It may have a different name if you're using different tag editing software). Copy the Feed field contents.

• Open your "bad" podcast files, paste/replace the "good" Feed info into the Feed field (they should be different - mine were) and click Update.

• Last important step is that you have to delete the "bad" files from iTunes and reimport them. If you right click or control click the file in iTunes before deleting, and select "Show in Finder," the enclosing folder will open. If you sort by date modified, all the podcast you just updated should be at the top, if they're in the same folder. Then you can delete all the bad ones, and drag them back into iTunes. Whey they're re-imported, they should now be grouped with the "good" podcast.