CMYK color space is a field where GIMP is a bit behind. But on the site “Linux for Designers” of Eckhard M. Jäger you can find two plugins and information how to use them. They are called separate+ and cmyk-tiff-2-cmyk-pdf.
For the reverse process, see working with PDFs in GIMP.
CMYK conversion is just one step before printing with ICC profiles.