Is it possible with Bonitasoft to let the user define the process step when initiating the process?
Processes are by nature deterministic.
I understand from your description that you want the end-user to specify what step to go next during a process execution. If there is a limited number of pre-defined options, it is possible because the process can gather input from the end-user (usually from a form).
An example of your use case would help to provide you with more complete answer.