I believe you might be able to customize Document Manager for OpenPlant (you can sure do it for PID, not sure for 3D).
However it is not a tool intended for that purpose. Also, as currently there is no common database for OpenPlant 2D/3D in a file based mode, there is not much value from having documents in a Document Manager or Vision.
Unfortunately, if not using ProjectWise, the best other way to use OPM is just having files with-in the Workspace \ Project folder (C:\ProgramData\Bentley\OpenPlantModeler V8i\WorkSpace\Projects\OPModeler_Metric\WorkFiles\Models\ by default).
Regards,
Justinas