The K-1 is about the thinnest you can have and still make it an extension ring, i.e. with bayonets on either side. The K-1 on a 20/3.5 will produce around 1:3 which is about as far into the subject you would like to go.
Anything thinner means you have to take off the mount of your designated WA and either shim it or remove the near-focus stop, or both. Most of the wide angles focus on their own to something like 1:5 to 1:8 (for modern lenses, older designs went normally only to 1:10). The new 20/1.8 Nikkor does a little better to about 1:4.
Somewhere between 1:2 and 1:4 should be within reach by the shimming technique, but to what purpose? Most likely one would increase lens aberrations, introduce spherical and chromatic aberrations, and reduce image contrast.
Also take into consideration that working distance (front of lens to subject) will be very short, or even negative meaning focus gets inside the lens assembly. The latter occurs if you combine 20/1.8 + K-1, by the way.
There was recently some publicity about a generic 15 mm lens from China, cannot recall the maker's name, but the lens allegedly went to 1:1 life-size; however, at this magnification, working distance was reduced to a few mm.
