separate +
Can separate+ be added to 2.8.22?
You really need to state the operating system when asking this type of question. The solution for Windows is different for say, linux and OSX another matter again.
Assuming Windows, you can try this how-to demo https://youtu.be/oQ5655I-wrY
You need the files from here http://www.mediafire.com/file/azx5y5uyd7t0a31/cmyk_utility.zip
Unzip it, there is a folder 'separate_plus_for_win7' inside that are these files + some others
icc_colorspace.exe libtiff3.dll separate.exe separate_import.exe these go in your Gimp profile C:\Users\your-name\.gimp-2.8\plug-ins
Explanation, the plugin nearly qualifies for ancient, needs that libtiff3.dll from Gimp 2.6
Does it work in Windows 10, a quick test was ok, see screenshots.
What are the other files in that zip, There is the separate conversion utility CMYKTool, have not tried recently might/might-not work in Win10 and a couple of icc profiles.
An even older demo on use here, but it is Gimp 2.8 http://youtu.be/2rby7r771D4
Alternatives, For something with a GUI try krita, command-line use ImageMagick
rich: www.gimp-forum.net