In the latest development release you will find a new wizard, the Scene Builder Wizard. With this feature you can build one scene (object) from a set of FS library objects and their placement. With this information ModelConverterX will generate a combined scene that contains all these objects at the right positions. This might be useful to improve performance (when applied on a small scale) or when you want to convert an entire airport to another format or so. Once you have made the new object you can export or edit it like normal.
The video tutorial below demonstrates this new feature.