Creating a Spot Color Channel in Photoshop

Can anyone teach me how to make a Spot Color Channel on Photoshop? I am using this image as an example. The blue color in the background needs to be printed mostly with a spot color that is close to the original painting color (ex: Pantone Blue 072C) . But the CMYK color still should be present in within the blues to resemble the subtle nuances of the brush strokes. I can’t figure it out on my own.

The reason I am doing this is because the artist’s works were represented on many publications over the years from many different countries. And we found out that CMYK color alone cannot duplicate the blue color very well (I mean the blue only, no problem with other colors). I know, it is hard to believe, but if you see all the publications and compare it to the original art, you would see the huge difference in color saturation and luminosity. That is the reason, I think it is necessary to use a spot blue for an upcoming catalog of the artist.
It will be a huge help in my work progress if I learn to make the Spot Color Channel. Then I can talk with the printers about this method.

Please see the attachment.