What makes a podcast a podcast according to the player is if the file has the genre podcast or if it is in the podcast folder. It can be a song or a spoken word file. If a file has the audiobook genre or is in the audiobooks folder, then the player considers it an audiobook, even if it is a song, or a spoken word file. If the player doesn’t consider a file a podcast or audiobook, and the file is an mp3, wma, ogg, or flac file, then the player considers it a song. To have podcasts in playlists if you want to put them there, just follow the rules the player has for what it considers a song. Keep in mind though that if a spoken word file is considered by the player to be be a song, then it will show up in song lists, and it will play when you shuffle all your songs. There won’t be any autoresume function for it. Playlists are just for what the player considers a song.
Instead of using playlists for podcasts, just give all the podcasts you want to play as series the same album tag, and assign track numbers to them so they play in the order you want. Then you could put this album of podcasts in the podcast folder. That is how I arrange my podcasts. Then I just Go to music>podcasts on the player and choose the album of podcasts I want to play, and it plays in the order I arranged based on the track numbers. I use the free program MP3TAG to make it easier to edit the tags.