This one has been bugging me for a long time. I'd googled for the solution, and found that others have had the same issue, but no one had found out how to fix it. I've figured it out, so I guess I'll post it here:

How do you manually add episodes to a podcast so that they show up in the "Podcasts" section of iTunes? You can add individual MP3 and AAC files by choosing [Add to Library...], but they show up as music and don't list with the other episodes of that podcast. Which is really annoying.

The problem I had was that I was missing a number of episodes from one of my favorite podcasts, Security Now.



See how episodes 66, 67, 68 and 69 are missing?

I can download them from GRC or TWiT, but they won't go in this list if I add them to iTunes. They'll just show up as music. And that bugs me.

Well, after messing with it for a while, I figured it out....

1. Find the URL of the XML file that the podcast uses

In this case, it was http://leoville.tv/podcasts/sn.xml

2. Setup a webserver

In OS X, you just "Enable Personal Web Sharing" in System Preferences.

The directory where you put the files is in /Macintosh HD/Library/WebServer/Documents/

3. Put the MP3s of the podcast on the webserver

In my case, the names were SN-066.mp3 through SN-069.mp3

4. Craft a fake XML file for iTunes to read

What you do here is take a look at the actual XML file, then make one that instead of containing entries for say, the last 20 episodes, contains entries for the old episodes that you want to add. Here's an example of what you would insert for one episode:

    
      <title>Security Now 66: Vista Security!</title>
      <link>http://192.168.0.10/podcasts/SN-0066.mp3</link>
      <description></description>
      <author>leo@leoville.com (Leo Laporte)</author>
      <pubDate>16 Nov 2006</pubDate>
      <category>Technology</category>
      <category>Security</category>
      <comments>http://twit.tv/sn66</comments>
      <enclosure url="http://192.168.0.10/podcasts/SN-066.mp3" type="audio/mpeg"/>
      <itunes:author>Steve Gibson with Leo Laporte</itunes:author>
      <itunes:subtitle>Leo and I describe the new security features Microsoft 
has designed and built into their new version of Windows, Vista.</itunes:subtitle>
      <itunes:explicit>no</itunes:explicit>
      <itunes:duration>37:45</itunes:duration>
    </item>

So you go through and add XML entries for each MP3 you have to add. Notice how the URL for the MP3 is changed to point to the MP3 hosted on our webserver here.

5. Change your hosts file

Change the hosts file (in OS X, it's at /private/etc/hosts) on your machine, so that iTunes will go to your webserver instead of the actual one hosting the podcast.

(Be sure to change this back when we're done so that it updates properly)

##
# Host Database
#
# localhost is used to configure the loopback interface
# when the system is booting.  Do not change this entry.
##
127.0.0.1       localhost
255.255.255.255 broadcasthost
::1             localhost
192.168.0.10    leoville.tv 

That's what mine looked like. 192.168.0.10 is my PowerMac G4. You could do this on your own machine if you wanted to by using 127.0.0.1 instead.

6. Now, update the podcast in iTunes

Instead of going to the actual website to fetch the XML file for the podcast, it's now going to your computer to get it.

And now you have the option to add those long lost episodes to your list:



It is incredibly annoying that there isn't an easy way to do this in iTunes. But in the meantime, I guess this works.