I have just "hacked" it to use my master dark created in PixInsight. Here is how i have done it (some steps are the same as you just mentioned):
1) Go to guiding tab of Ekos and enable Dark checkbox
2) Click on Capture button. This will create one dark
3) Go co Capture tab, select your guiding camera. Create sequence for 20 (or more if you want) darks with the same exposure time used in first step. Dont forget to select Type: Dark. Execute the sequence
4) Stack these 20 captured darks in your favourite software. Resulting image must have the same bit depth as single sub (in my case it is 8bit)
5) Possibly optional step: change content of FITS header for stacked master darkto be exactly the same as has the one dark from step 2
6) Go to /.local/share/kstars/darks/ folder and rename the original dark from step 2. For example from "darkframe_2021-01-21T20-28-28.fits" to "darkframe_2021-01-21T20-28-28.fits.old"
7) To the folder above copy stacked master dark and change its name to be the same as the dark from step 2 (in my case "darkframe_2021-01-21T20-28-28.fits")
Go to KStars/Settings/Configure KStars/Ekos/Dark Library. Now if you double click on the record for dark from step 2 it will open the stacked master dark.
Most of guiding cameras are uncooled, so take the darks in step 3 in similar conditions as you will have during real-time guiding.
This is still a theory, I currently don't have conditions to test it outside, but I will report once done so

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