It doesn't have a hard drive, it has 512MB of solid state memory. I really doubt there's enough room inside that case for even a 2.5" hd.
As for external hard drives, It will probably only take a firmware upgrade to allow the Wii to support them. However, it doesn't support them now, and there has been no announcements saying that it will in the future.
You're basically asking us to forsee the future here. Who knows.