To be honest, it looked to me as there is only one external call possible at the "ExternalWFStubs.MachineProvisioned" stub, but what if i can create another stub based on this provisioning state? Something like "ExternalWFStubs.MyMachineStateEqualMachineProvisioned"? Heureka!
Here is how:
First you have to install the CDK plug-in for Visual Studio. This enables the vCAC workflow generator and allows you to generate a "StateChange" workflow with your own stub name.
I named my workflow: "MyShinyWorkflow" and this is shown as naming in the solution explorer.
Now you can build everything you want in Visual Studio... or you can upload the empty workflow in your vCAC like described in the .txt file (just open the solution in a windows explorer and copy the files).