You could do it with matlab (or whatever the language)

f = imread('ngc6543a.jpg');

f = im2double(f);

k = 1-max(f,[],3)

c = 1-r;

m = 1-g;

y = 1-b;

CMYK = cat(3,c,m,y,k);

imwrite(CMYK,'cmykimg.TIFF');

You could also used imagemagick with the commande line

magick imgrgb.png -colorspace RGB -colorspace CMYK imgcmyk.tiff

Hi Maxime, thank you!

Actually the command was: convert mgrgb.png -colorspace RGB -colorspace CMYK imgcmyk.tiff

I didn't know this tool, it could be very useful for a lot of other options too! 

'convert' is for previous version of imagemagick.

Yes it is a usefull tool. I also use it to make .GIF for my presentations.

Mmmm ... I installed it by macports v. ImageMagick-6.9.6-2 and only convert was working ...