Q: How do I transfer podcast episodes onto playlist?

Do you have a playlist in your iTunes library that syncs (automatically) to the iPod when you connect it?

If so, your question is about updating playlists in iTunes (in general), correct?  Not specifically about updating podcasts on iPod, because that part happens automatically when you connect iPod.

There are a few ways to add to playlists.  One way is to show the source list (such as your podcasts library) in the main part of iTunes window.  In the (left) sidebar, make the target playlist visible (without selecting it).  Drag and drop podcast episodes from the source list to the playlist (in sidebar).

You can also select the playlist in sidebar, so that is shown in main part of iTunes window.  Near upper right corner, click Edit Playlist button.  The playlist moves to sidebar along right side of window, and main part of window becomes the source list.  Drag and drop from source list to right sidebar.

The third way is to select item you want to add, right-click it, and Add to Playlist -> [name of playlist].

NOTE:  Podcast episodes must be stored locally (on your computer's drive), if you want to add them to playlists.  I noticed that episodes that are only stored online (for streaming) cannot be added to playlists in your iTunes library.  Episodes that are not stored locally have the cloud with down-arrow (download) symbol.  You can click that symbol to download.