How do i achieve this ?
You could try this, but they are experimental:-
If you’re looking to try the separate+ plugin, this blog post and comments might be helpful.
you can do it with nodes, i think. yellow is the red and green passes added and ramped. (use the separate YRGB). B is just Y, M is R&B, and C is G&B.
ImageMagick can convert from rgb to cmyk:
convert yourRGB.jpg -colorspace CMYK yourCMYK.jpg
Can you post a setup ¿ and , mean in gimp, can you explain more ?