Enhance the core
From Modelio Community
It is possible to contribute even to the core of the Modelio Modeler itself. You should be warned that doing so usually requires a deep knowledge of both the UML specifications and the Java programmation language.
Set up the build environment
In order to enhance the Modelio core, you have to set up the Modelio development environment. You will find the tutorial describing the process to set up this environment at the following address:
Reporting issues and providing patches
To be provided.