Hi, is there any method to convert PNG output files of GIMP and Inkscape to CMYK using Krita ? Thanks.

Yes, just open the PNG in Krita and convert to CMYK... but then you cannot save it in CMYK because PNG doesn't support it. Try JPG instead? (with 90% quality it should be fine enough, provided you don't save over and over again the same file, but you shouldn't do it anyway...). (Note that Krita saves proper color model + profile with no quality decrease).

Also what kind of CMYK? Are you sure you have the appropriate profile? The most common ones are Fogra and SWOP; note that Krita doesn't have either (Chemical Proof is not good for the most common usages; Fogra and SWOP are both closed, so Krita cannot contain them by default), so you would have to download it (for free) from the internet (if you know which one you need) or, best, get from the publishing/printing company you will be printing with. Nowadays you should be able to provide an RGB file and the printing company should take it; and it's the recommended path for people who are not well-versed in color management.

Thanks, very helpful. I simply want to print a business card design through a printing provider. In that case, which profile would I need ? I think Google will point to the CMYK profiles you mentioned. I am not sure whether RGB is accepted by local providers. At times they are too non-technical than I am. They depend on Adobe tools.

You need to ask your printing provider some questions:

Which file types do you accept? (.jpg, .tiff, .psd, etc.)
Do you have a preferred file type?

Will you accept a file that uses the sRGB colour profile?
If you prefer or insist on a CMYK colour profile, which CMYK profile do you use?
In case of CMYK, can you provide me with the profile or do I have to search for it on the internet using the profile name that you give me?

Here is just one possible source of CMYK profiles for printing: https://www.color.org/registry/index.xalter