I think the problem is not the missing physics scene - Omniverse should create a temporary one if there is none.
I forgot that I had to implement a custom OmniGraph node which needs to be installed as an extension. Unfortunately I may not have the node anymore, it was just an Experiment.
Here is what I did:
Create a curve in Blender to represent the motion path.
Export/import the curve to Omniverse via USD.
The curve is now a mesh which is invisible in the viewport as Omniverse does not yet support curve.
The mesh object contains the curve as an array of coordinates in the „points“ Attribute.
Use OmniGraph to read the point array and update the sphere‘s translation according to the coordinates in the array.