actually, you can do this with mp3 files through itunes without going through a messy conversion process.
1. highlight all of the mp3 files that are your audiobooks.
2. right-click, select "get info".
3. under options, go to media info and select audiobook and save.
itunes will then put the audiobook tag on those files and they will show up in the audiobook section and function like an m4b file.
unfortunately, i don't think there is a way to do this with podcasts. there isn't a podcast section file type selection, and i download my podcasts with another program.