Dear all,
after lots of investigations and trying to reach somebody that could help, finally an awesome user "hattrick" from the Musiker-Board forums found the USB Stick and provided the V2.01 Software to me and it worked! Now, the SuperNATURAL Piano Kit (K-RD700GX1) works on my RD-700GX and I am enjoying the awesome piano sounds. To contribute to the Roland community, I will provide the ZIP download for everybody having the same issue, please get it here:
https://drive.google.com/u/5/uc?id=1g64i4ms4t1IZvXDVD5JTl7fYSuDisA8M&export=download
With this software version 2.01 the Roland RD-700GX must be updated and only then it can access the new sounds from the SuperNATURAL Piano Kit. Just extract the files from the ZIP onto a clean USB stick and install it. (Check manual from the Roland Website)
Now, the community can reduce the frustration about the bad Roland support, they did not want to take a few minutes to check their software archive and provide the software via their website, instead they took the minutes to write me why they won't give support for a high-end product which they sell as solid long-term music equipment for the stage. The RD-700GX is still a high-end stage piano and a long-term investment, for this Roland should care about their customers and provide a bit of support to increase their customer satisfaction.