You can kind of do this already although it's not ideal.

If you create a folder in either the \BallisticNG\UserData\MyShips\ or \BallisticNG\UserData\MyTracks\ directories it will act as a category in the ship/track selection screen.

The downside of doing this is that you will have to manually move everything over from your steam workshop folder* and you'll no longer receive updates as a result. I suspect that tracks may need to be in the workshop folder in order to run in online multiplayer too so that's also something to bear in mind.

Hopefully this is something that will be looked at for 1.0 but, for the time being, this might be of some use to you.



*
\SteamLibrary\SteamApps\workshop\content\473770\
Run a search for .wtrk files for tracks and .shp files for ships.

I see, still thanks for the tip.