You have to attach a CMYK icc profile first (I'm not sure if it's a
requirement, but if I remember correctly you have to save and reload
in order to apply it).
Then, going to CMS tab in the fill/stroke dialog you can set the CMYK values.
The resulting SVG file can be opened (mostly) in Scribus 1.5.0trunk,
and the CMYK values will be preserved (you can check in the colors
dialog, that the imported colors are in CMYK mode instead of RGB as a
Scribus doesn't fully support Inkscape SVGs yet, but as far as I know
that's already on the works (you can tell by trying Scribus nightlies,
SVG support has been improved a lot).