Registration Black, (CMYK Black), and CMYK White are protected colors in Scribus.
Personally, I think that this limitation should be removed, but I'm not sure that it will ever happen.
For now, you can just ignore those colors: if you don't use them they will not affect your exported document.
moved from comment
Microsoft products are notoriously bad where color is concerned. Their support may have been less than forthcoming. You do NOT want to alter Apple profiles to fix some Microsoft Office issue.
Everything in Office uses RGB color. RGB has a habit of changing values based upon which computer is opening the document. You simply can't control ...