There was a cluster version 4.1(3) which was upgraded to 4.2(3).
Now I have CCM installation CDs version 4.1(3) and upgrade pack to 4.2(3).
Can I add a new CCM subscriber 4.1(3) to this cluster? I guess it's impossible.
Your suspicion is right on, Alexander. The installation of the subscriber will require the new subscriber to be the same version as publisher, otherwise the process will fail with 'version mismatch' error.
One option for obtaining the required software is via the Product Upgrade Tool (PUT), where you would enter your software contract and request the version you need.
Hope this helps.