If you have not already, you can map the FloorSpaceJS space types to standards space types for DOE or other prototype building types, and then run one of our "create_typical" measures such as this one for DOE building types. This measures expects a model that has stub space types tagged with the standards building type and space type. It has individual switches ti add constructions, internal loads, SWH, elevators, exterior lights, exhaust, refrigeration, internal mass, and HVAC systems. Mechanical systems can be specified or inferred from high level inputs. https://github.com/NREL/openstudio-model-articulation-gem/tree/develop/lib/measures/create_typical_doe_building_from_model

Here is an example OpenStudio Workflow (OSW) that uses a measure to convert FloorSpaceJS to OSM, and then to add constructions loads and systems. I did notice your question talks about adding HVAC before importing from FloorSpaceJS, but what I props here is the opposite. The FloorSpaceJS schema doesn't currently contain inputs for mechanical systems, other than just the thermal zone object.

One Additional note, the FloorSpaceJS schema does have space types and a name field, but doesn't have standards information. You could modify the FloorSpaceJS translator to infer standards space type from the space type name or from a lookup table. In our case we are merging back into an OSM seed that has already identified these space types contained in the floorSpaceJS jason input file.