Ok I'm got a lot things done by now. I can write hmp5 files right, but they seem to be shown not really right when I load them in GS. If I convert the files in med to hmp7 everything is alright though.
So I have a question belonging the hmp7 format:
I think the only difference between those two formats is the vertex format. Here the difference in the class:

From HMP7W:
bool Vertex(short z, float nx, float ny, float nz);

From HMP5W:
bool Vertex(short z, int normal_index, int pad=0);

I think nx,ny,nz means the normalinformation of that vertex. Is that right? So the only thing I have to change for righting to HMP7 is to use the HMP7W class and give the normalinformation in the vertex struct?