Sorry, I did not read the part about combining it with the Hokkaido Rail Pass.The "interchange station" is Nakaoguni, a minor station at the tip of Honshu. But the trains between Honshu and Hokkaido do not stop there. Therefore, I doubt that you can do the trip by combining the two passes. There is a high risk that you would have to pay a supplement for the section between the last station served on Honshu and the first station served on Hokkaido.
I also recommend to use a Japan Rail Pass instead.