Here is a set of pattern Tones, like the ones used in mangas…
Download Here

The best way to use them is to fill a layer with it, apply a transparency mask to it, erase all and draw on the mask. ( FEATURE REQUEST :It would be nice to have a way to initialize the transparency mask to "All transparent" with one click !.. )

What do you think of adding them to default patterns (or at least some of them) ?

Until we get a Tones brush engine, maybe some day! (Hatching and Grid brushes can already do some, but can't do all of those patterns yet).

Cool! Now I have to figure out a way to turn them into .pat files... Maybe gimp can do it . As for the feature request: yes, that's something to look into.

I didn't know the .pat file format. I've imported those .png one-by-one using the "+" on the pattern panel, it worked nice!

If you really need to convert them to .pat then as you said Gimp can do it.

Although apparently Gimp's .pat is different from Photoshop's .pat : "Caution:Do not confuse GIMP-generated .pat files with files created by other programs (e.g. Photoshop) – after all, .pat is just a part of an (arbitrary) file name.(However, GIMP does support Photoshop .pat files until a certain version.) "

Krita can import png files. Just put them into the right folder.