Exporting a file in CMYK or RGB
 

I need to provide a CMYK file from a graphic I've created in the current version of OmniGraffle Pro for a printing house. Very soon. What are the steps to produce a CMYK TIFF export file ? One can easily choose between many output FORMATS, but I can't find a feature that allows to specify CMYK or RGB in the export file.

Also - is it possible to export an existing graphic file from OG strictly in RGB?
How does one go about exporting a file in CMYK (or in RGB) color system?
Would greatly appreciate your advice!
SvetaG