since this feature didn't made it in S3D 3.0 I tried an other way to get this.
I cut a part of the 3D-Model out which I wanted to print with 100% infill (Image #1). Then I moved it in the whole model to the right position and created a process for the small part with 100%. But when I sliced it, the slicer prints the infill of the big part in to the 100% infill part (so it is printed 2 times, Image #2).
Then I cut the small part out of the big part and tried it again, but then I get a gap between those two parts (Image #3).
So I have still no perfect solution...