How to insert large areas such as a nature park?

It might be easiest to make the shape file available to the community for someone to add it properly. There will likely be areas in OSM already - eg an administrative boundary, a residential area or so - where the boundaries coincide partly with the Naturpark boundaries. Simply adding the Naturpark shape will inevitably mean that boundaries overlap or have gaps. "Doing it right" often requires identifying and re-using existing linework for the new boundary, which can also mean splitting existing linework to re-use only the parts needed. This is one of the most difficult editing operations in OSM and requires some previous experience to do right.