I found some way to achieve, what I wanted, using the maven-dependency-plugin.
The basic problem was, that no code should have been duplicated, and yet, the same module should be built with additional/replaced content on demand.
The way I did it, was to make a second module, and within declare the module I wanted to modify as sole dependency. Then in the build cycle I added the maven-dependency-plugin with goal "unpack-dependencies" and selected to unpack the original dependency into the target/classes folder at "prepare-package" time.
That way I got a second module, which had the content of the first module, plus some modifications I could then make to the main sources of my adjusted module.
Altogether this is still everything but pretty, in my opinion Maven had its time, but now it's time for a better build tool ;-)
By the way, if anybody knows a more elegant solution, I'm still open... My problem often was, that under no circumstances I wanted to duplicate my configuration or even my code. And it seems that Maven makes this really hard...