After usual wait of one month I got pen adapter ordered on Aliexpress:
I chose this model because one can change tip position by screwing in or out part with cylindrical protrusion. This gives some versatility, but also creates a few challenges. First of all counter screw keeping part with protrusion in place after regulation has a bit to big diameter and does not allow to fit the socket in plotter as it should. But it does not matter that much, because it's presence makes impossible to take out adapter from socket in the first place.
To take out pen from the socket, plotter head lowers bottom plate (it's mounted on a spring), grabs pen and pulls it down (socket has protrusions keeping pen in place on beneath of a saddle). Counter screw does not allow to lower the pen, thus it's impossible to pull it off from the socket (at least in case of my plotter). But it can be removed, as you can see on the photo above.
Last challenge to deal with is to find the perfect distance between cylindrical protrusion keeping adapter in the socket and tip of the pen. First few experiments show that it's not that easy as it sounds. As for right now I was able to make plotter head grab the pen (although there is still space for improvements), but putting it down is still problematic. Moreover, absence of counter screw crates some problems as it's not easy to make small adjustments without ruining position by an accident.
To resume I'm happy with this buy as I will be able to easily experiment with different dimension and then switch to designing my own 3D printed adapters.