I have updated ModelConverterX now so that the additional PBR material options that Prepar3D v6 offers are also supported. There are a number of new material properties related to precipitation effects (including a new precipitation texture) and some other new material flags as well. Besides that P3D v6 also supports exporting vertex colors, so that has also been added to ModelConverterX.

You still need to export as P3D v4.4 MDL file from ModelConverterX (the MDL format is still called PV44), but you need to use the Prepar3D v6 XtoMDL to be able to use the new options.