Hi Eduard,
schHiCreateWireStubs() won't expect the instTerms to be there already - if they were in a normal schematic it would be because the wiring is already present - and so no need to create a wire stub.
So if you want to add wire stubs, you'll have to use schCreateWire() to do it yourself.
As I said, the key thing is that you have to follow the schematic editor's semantics if you're going to use the schematic editor to edit the design.
Regards,
Andrew.