No Aperture does not have the internal smarts to do what you want. You can have it store the originals on an external disk but not to pick and choose where to store a file depending on its type.
If you are not importing lots of images at one time doing it manually would not be to bad. You would import everything into the Aperture library initially and then using a filter or smart album collect all the images of the type that you want to have on the external drive and then use the Aperture command File->Relocate Orginals...
You should certainly look into using Aperture with Referenced Originals. You store all (or as many as you want) of your imported files on your external drive, but keep the Aperture Library on your system drive. You need to have the external drive mounted, and the Originals on-line, in order to make adjustments to your Images and to print them or export full size files, but when your Originals are off-line you can still assign and change metadata, organize the Images in your Library, and export the Previews. This is the standard configuration for anyone with a laptop or any machine for which the system drive doesn't provide enough storage space for the imported-into-Aperture files.
Read this section on the linked page in the User Manual for some more information:
You might also find my short introduction to the parts of Aperture helpful in making your decision.