You can open the palette docker separately too and selecting a colour in specific colour selector will also make it your foreground colour then you can add this colour to the palette just by clicking on empty cell in the palette docker.
Another work around, which might be cumbersome to you, but if you have to use the colour selector from the top bar no matter what, then there is a way. Most input boxes in Krita support math operations. so you can input something like 255/2 or 255*0.5 to get 50% of any colour, or say 255*0.35 to get 35% etc.
