Could not find anything about this in the documentation.
What I want to achieve is to copy a file into a subchart's directory which will be used by the entrypoint of the image.
The problem what I want to solve is to avoid subchart unpacking and copying the file by hand into that.
From the Helm point of view, there are Helm Chart Hooks. They allows you to execute custom images (with custom commands). You can check pre-install hook.
For your use case though, since you want to execute something before really executing the Helm command, you may want to end up with a bash script which executes something before helm install.