The syncronization between Google Reader (web) and Google Listen (phone app for listening podcasts) is limited to podcast subscriptions and marking episodes as read/listened and unread/unlistened. There is no direct way to add episodes to your Listen "My listen items" queue from Reader.
You can still access old episodes from Listen by scrolling down and clicking repeatedly on the "Load more episodes..." link at the bottom, and you can add them to queue from there. Of course, this isn't as friendly as selecting them from Reader.
However, here's a workaround that might work:
- In Google Reader, add a specific tag of your choice to the old episodes you are interested in.
- In Google Reader, go to "Manage subcriptions -> Folders and Tags" to make your tag public.
- Access the public page of the tag to get its Atom feed. That link should be a valid podcast feed containing only the episodes you have tagged (even from multiple different podcasts).
- (This is the step I cound't get to work) Somehow subscribe to that created feed from Listen. Since you can't add a tag directly to a folder in Reader, you need to subscribe to the Atom feed of the tag, and from "Manage subscriptions" add that feed to the "Listen Subscriptions" folder. However, I coulnd't get Listen to show any items in that subscription, even though it looks ok when viewed in Reader.
These questions might also help: