A good point does separate+ work in a 64 bit Win 10 and Gimp 2.8.14 - yes, at least it does here.
If you look at the bottom of
http://cue.yellowmagic.info/softwares/s ... index.html there is a package that includes/
does not require lcms. separate+-0.5.8+lcms2_win32_bin.zip
In Win10, opened a file, converted to cmyk and this the export dialog.
http://i.imgur.com/ZGCMnjP.jpgSeparate+ is a 32 bit plugin, still works, pop it in your Gimp profile.
http://i.imgur.com/gjbG0sR.jpgAFAIK the partha version of Gimp includes separate+ you might want to try that.
A couple of years ago I bundled it all up for someone.
On my mediafire account
http://www.mediafire.com/download/azx5y ... tility.zipand a video to go with it
http://www.youtube.com/watch?v=2rby7r771D4&hd=1If you only have one or two images to convert and not too large (< 12 MB), there is a free online conversion at
http://www.rgb2cmyk.org/