Converting CMYK to spot color using Illustrator's Live Color feature
Open the file to be converted.
For this tutorial, I've selected some 3D targets that I want printed in two colors: Black and a reddish pink. Check your color palette to see what you're starting from.
Everything is already in CMYK. The two shades of gray are just screens of black, so that's good to go. The reddish pink, however, needs converting.
First, select all instances of the reddish pink color.
Then, in the menu bar, click "Edit" --> "Edit Colors" --> "Recolor Artwork"
This brings up the Live Color palette. Click the icon that is a small six-rectangle grid to bring up a list of spot color libraries to convert to. When in doubt, choose "Pantone Solid Coated."
Click the little box in the lower left of the window that says "Recolor Art."
Click "OK." The Pantone color that Illustrator judged to be closest to your original CMYK values has filled in your art and been added to the document's color palette.
You should be able to safely delete the old CMYK color now, and now this document is two colors: Black and Pantone 206 (spot).