To load OpenFlight you'll need to use osg2vsg to load the data as an OSG model then have osg2vsg convert it to VSG.
If you build and install osg2vsg, then build vsgXchnage it'll add osg2vsg::OSG ReaderWriter to the vsgXchange::all, or you can add the osg2vsg::OSG ReaderWriter to your own vsg::Options.readerWriters list.
Longer term it may be appropriate to write an OpenFlight loader specifically for the vsgXchange, perhaps even just port the OSG's one.
For a vis-sim you could convert the OpenFlight models to the VSG's native .vsgb so the runtime wouldn't need vsgXchange/osg2vsg/OSG.
Tasks like a dedicated vsgXchange::OpenFlight loader will be something that is community led, so far no one has come forward to write or fund development of it. What has been developed so far is a reflection of what has been seen as priority to existing VSG users.
Cheers,
Robert.