Using Krita 2.9, Linux or Windows 8:
I open a flat RGB image in Krita and change its color space to CMYK using SWOP profile. Then I separate the image into its C, M, Y, and K components. At this point, I have 5 layers, including the original un-separated CMYK image.
I edit one of the layers. So far, so good.
Now, I wish to re-compose the image. Instead of separated layers, I want the image to be unified, in a way that incorporates the edits I made to one of the separated layers.
I cannot find how to re-compose the layers in Krita. Can this be done? It does not matter if I first delete the original layer, or not.
Note that the original image is not updated to reflect any change to a component layer. And, exporting to various formats (PDF, TIF, JPG) merely reduces everything to the topmost visible layer.
By comparison: The infamous separate+ plugin for GIMP will not only separate to CMYK layers, it will re-compose them with edits in effect.
EDIT: I discovered that GraphicsMagick can help me (outside of Krita). It can separate the CMYK image into its four channels as separate greyscale images. I can edit the separated images as greyscale, then re-assemble them to a single CMYK image, again using GraphicsMagick. The procedure is described on their website.