No, that James Morrison mentioned in Isaac's will was unrelated to either of them. He appears to have been a prominent land speculator in the Ohio Valley area who came from the Mississippi Valley area first. I read an article on him in a geneaological publication a few years ago.
There was also a Samuel Morrison living near Baskingridge/ Morristown NJ during the Revolutionary War who had a son of the same name. Do you know anything of his family?