Bad news first: It is not possible to directly save a PDF from Inkscape to CMYK color space without installing an extension. However, if you are OK with running a single command from a terminal, you may simply convert any PDF from RGB to CMYK color space.
So, save your PDF from inkscape as you are used to, e.g. by selecting
File -> Save a Copy ...
from inkscape’s main menu and chossing Portable Document Format (*.pdf). Subsequently, let’s assume your file is called document_rgb.pdf.
For converting the PDF from RGB to CMYK color space, we use ghostscript. ghostscript is most likely installed on your Linux computer (it is available for Windows as well), and the conversion command reads as follows
gs -dSAFER -dBATCH \ -dNOPAUSE -dNOCACHE -sDEVICE=pdfwrite \ -sColorConversionStrategy=CMYK \ -dProcessColorModel=/DeviceCMYK \ -sOutputFile=document_cmyk.pdf \ document_rgb.pdf
The output is saved to a file called document_cmyk.pdf.