Yes, I finally got it, and this is as far as I know THE way to generate a swatch-file for your newly-created shader.

This swatch file is handy to represent your shaders in your visor or your hypershade by picture.

In hypershade:
1) Save your shader:
File-->Export Selected Network

This shader will be given a default image, which has the name "maya" on it.

2) Make a render

In renderview:
3) View --> Grab Swatch To Hypershade/Visor
Select an area of your render...
...and move this pic onto your default image in hypershade (and probably visor too?) with your middle mouse button.

Maya will then assign a catalogue under your shader catalogue, containing the new swatch file, which tells maya how the shader looks like in you shader-catalogues.

(!) If you render out a BIG image for your swatch, then you'll end up with a BIG swatch file.
I noticed that maya's basic shaders has a swatch-file-size of about 17 Kb. It looks that 100x100 pixels renderings give the same file size as maya's basic swatches.