If Rhythmbox knows about your podcasts (i.e. they show up on your Music list, being in your Music directory, with the Podcast genre, but not necessarily in your Podcasts tab), you can do the following:
From Banshee, go to Media -> Import Media. From the menu, choose "Rythmbox Music Player". This will import all of the tracks that Rhythmbox knows about, including your podcasts. They won't show up in your Podcasts tab, but they will show up in your Music list under the "Podcast" genre.
If these don't show up as normal tracks in Rhythmbox, you can do the following to get them to show up:
- Move your podcasts directory inside your Music folder.
- Wait for it to be scanned or close it, delete .local/share/rhythmbox/rhythmdb.xml and then restart it. (Warning: This will wipe out your radio stations & possibly other things, so save a backup first!)
- Podcasts should now show up in your Music with the Podcast genre (assuming that they were tagged correctly).
- Give Rhythmbox and Banshee the new location of your podcasts directory (inside your Music folder).