You can add pre-existing TV Shows from a different location by dragging into iTunes while pressing the Option key - it'll reference the original file. But this doesn't work for new shows or at all for podcasts. It'll create a folder for each of the TV shows or Podcasts at the artist level when downloading a new one.
I gave up trying this for individual items and just moved my whole iTunes Music folder onto an external drive and created an alias to it.
iTunes does not like Aliases created in the finder. Seems to work fine with Symbolic Links in my experience. Have been doing this this v.6x, has been stable.