Hi Lionel,
If you buy the standard 7 day pass, it will be valid in Hokkaido, so you should be all set. The only thing is that it is a *massive* journey.
Getting the sleeper is a good idea, but how about mixing some of northern honshu into the trip as a stop over like Aomori or Akita? There is some amazing scenery there, and also some pretty good national parks like the
Shirakami Sanchi World Heritage Site.
Hope this helps!
J
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Japan Rail Pass - buy the JR Pass online