It was not so difficult to enable this feature. There is a proposal on how to enable MIDI cc for VST3 here:
https://github.com/olilarkin/wdl-ol/issues/126.
Based on this example I have enabled channel aftertouch. For those who are interested, you can find the solution here:
https://github.com/pressplay123/wdl-...77f22fe32a7eee.
Summary: You will have to add parameters to your VST3 plugin and map the midi control messages to them. I recommend making hidden parameters (ParameterInfo::kIsReadOnly), so they are not visible and subject to automation. In a host, you can automate the midi aftertouch directly, so there is no need for another visible parameter.
The solution has a drawback: since the IPlug lib has to be modified, the changes will apply to all plug-ins using this lib. So you have additional parameters, no matter if you use them or not. Removing already initialized parameters does not seem to be supported by the API.