Hi Jean Paul,
this behavior is fully supported by Mendix. Within a microflow you can retrieve/create/manipulate any object from any entity in any of your modules (except for some entities in the System module)
It is also possible to associate an entity from module A to an entity in module B with the same capabilities as within the module itself (these are called "cross module associations", see https://docs.mendix.com/studio/domain-models-association-properties)
You could either use a reference or use inheritance. Depends a bit on what you want to achieve. With a reference you can set this refernce between entity X in module A towards entity Y in module B. With inheritence you have all the attributes of the entity which you inherit from. Take for instance the filedocument object in the modeler.
Many thanks for Ronald and Joost to point me in the right direction: cross-modules associations or inheritance, exactly what is needed!