I know this, but I whish it was possible to do it in recolor artwork.

I have an artwork with a lot of gray colors in a CMYK document but many of them are with rich black and I want to convert them to K only. I can't convert all objects because some colors should stay unchanged and finding each color I need and then using select the same  will take forever while in the recolor artwork window I can easily see which color I need to convert. Is there at least, a way to select objects with a certain color that I see in the recolor artwork window? Or is there any easier way to do this?

And I don't want to substitute with some gray color, I want the same K value for a color that I get when I do Convert to Grayscale from the Edit menu.

But this will change all colors to one. I don't want this, I want to convert each of the desired colors to grayscale.

I'll give a simplified example of the challenge. Create several objects in a CMYK document using the HSB sliders with S set to 0 and color some of the objects with different grays and also make some of the objects with saturated colors. Select one object with a gray color and convert to grayscale and then to CMYK using the edit menu and this will make the color only with the K from CMYK. This is what I want to achieve for each grayscale color in a much more complex graphic with hundreds of different colors, some being rich grays, some being only K, and also some are saturated colors. I can find easily the rich gray colors by looking at the list in the recolor artwork but how can I convert them to K only? The challenge it to select the objects with those colors, then I can use convert the grayscale and CMYK from the edit menu.