Well I am not a unity developer, but I have looked a bit on youtube and how to mod unity, I assume that would be the best place to start. There are a bunch of tool, and things that exist for extracting assets from unity games, and they seem to work with 3DX as well. I suppose what would be nice would be a community mod that provided a way, and documentation on how to add new content, and was updated with new things as the community created them.