You don't need to convert anything. Just move your Raws to the External drive (File -> Relocate Masters). Your Previews will remain on the internal drive and be available.
Other than that, export the files from Aperture as Jpegs. Delete the masters from Aperture. Import the Jpegs again. Then ask yourself why you're using an app specifically designed to preserve your original image when that's not what you want.
Your Previews will remain on the internal drive and be available.
Only be sure to set the preview size to a good quality and high resolution in the Aperture Preferences and update the Previews before reloacating the originals. And you will only be able to edit the these images when you mount the external drive with the original raw image files.