Is it possible to graft a walnut scion onto a maple tree?
The book, Plant Propagation edited by Alan Toogood (American Horticultural Society/DK Publishing 1999) says that Juglans regia and Juglans nigra, grown for their edible nuts, are usually whip-and-tongue grafted. You would “use a slightly narrower scion than the stock so the thinner scion bark will align with the stock’s cambium more easily.”
I was not able to find any information on grafting a walnut scion onto a maple, but here is an article (pdf) on by William Reid, which has detailed information.
This publication from the University of Georgia College of Agricultural & Environmental Sciences, entitled “Propagating deciduous fruit plants common to Georgia” (1999) indicates that whip grafting or ring budding will work best for walnuts.