Whe I go to the Import window I do see my external hard drive on the list of drives, but the files do not show up.
Aperture will not show you files in libraries in the import panel. iPhoto libraries are not imported using the import panel. Importing the files using the import panel would result in importing a mess of thumbnails, previews, and originals, and you would lose your organisation in events and albums and probably most of your metadata tags.
You import iPhoto Libraries as library:
File > Import > Library
If the library you are importing is referenced, it will be imported referenced, if it is managed, the images will be imported managed. You can relocate the managed originals to your external volume directly after import using the command "File > Relocate originals"
But since there is a shortage of disk space, the OP should first open up each iPhoto library in turn with Aperture and do the relocate originals command to create referenced libraries. Then when all the individual libraries have been changed to referenced libraries, they can be imported one at a time without adding the extra space requirements to the internal drive.
A good point, Thomas.
I assumed, there is no shortage on disk space on the external drive, only on the system drive. So on the external drive the libraries can be united by importing into each other, even as managed. Merging libraries will require a lot of extra space, so it might be a good idea to move all libraries to the external for this procedure. Then finally, make the merged library referenced by relocating and move it back to the internal drive.