Is there a way to reverse placing a PTV Vissim project under scenario management? Or in other words, is it possible to export a scenario as a standalone *.inpx?

Click File > Scenario Management > Project Structure, tab Scenarios, select a scenario in the list, and click the button 'export selected scenarios' (floppy disk with the red arrow). This will export a standalone network file of the scenario (as an *.inpx). For more information, click Help > PTV Vissim Help and refer to the section 'Editing the project structure'.