I may not be understanding your question and I probably don't have the answer. Please bear with me.
First, you could just spawn the new process with
OS.execute, figuring out the arcana of the perfect command line. You should definately check the FeatureSet to be sure you can. In general, anything you spawn will be limited to your current permissions.
While I am unclear about Godot, the usual issue in spawning an interactive process is the keyboard buffering. You may find that something is buffering keys and only passing them on end of line.
Good luck! Come back and write-up your experience.