sour cherries are self fertile and do not need pollinizers. They also do not
pollinize sweet cherries

I'm thinking of
top-grafting his S.N. onto my Napoleon but I'm not sure if it's possible.

"top" grafting refers to grafting at the top of a standard, as opposed to
"bottom" or bud grafting, which is grafted at the base of the plant. This
wouldnt be possible because your plant has already been grafted. You could try
to graft buds from the other tree on you yours, but it would be way easier to
spend 30 bucks and get a second tree.

Toad