...the only thing you should do with a CMYK file is print it: CMYK is an output format. There is no single CMYK to RGB conversion algorithm (nor RGB to CMYK) because CMYK are ink colours and the results depend on exactly what inks and paper are being used. Also, proper RGB to CMYK conversion is not reversible because there can be more than one RGB value for every CMYK. Even PhotoShop struggles with this; if you convert RGB - > CMYK -> RGB you'll find that the first and last don't match by a significant degree.

OTOH, X could import CMYK TIFFs which could be used as-is on the page (and maybe separated). The on-screen appearance would be an approximation but any output (e.g. EPS, TIFF) would contain the original CMYK values. X might also export CMYK TIFFs from vector drawings where the colours have been defined as such. There would be limitations because many of X's effects (transparency etc.) are based on RGB.

Performing a professional quality RGB to CMYK conversion would require duplicating all the options in PhotoShop and is probably a feature too far. X has a tolerable RGB to CMYK conversion (which you can access via my x-Port CMYK plug-in in 'Host' mode) and that could also be improved and made more accessible.

Regards - Sean