You can use the CatchUp function to download episodes that aren't currently in your feed, be it that you haven't subscribed or simply because they are too old to fit in the feed.

In order to start catching up, first go to your account settings and select the option "Catch up to my public list". This is required since the list system isn't fully implemented yet.

Once this is done, you can head off to any show listed on the site and select the episodes that you wish to download. Finally, click the "CatchUp" button with the loop/replay icon. The episodes that you are currently catching up on are listed with a blue background color in the listing.

It is not possible to "un-catchup", but that feature might be added in time if it is requested :)