Magos' Model Editor
If a.) the model/s present an error regarding a ribbon emitter upon opening, or b.) the models go blank in-game after editing them in Magos', use MdlxConv to convert the uncorrupted file to MDL, edit the model as you wish, and then convert back to MDX before importing. If c.) the model/s refuse to open in MDL due to particle priority planes; remove those lines using a text editor (search for those keywords) and restore them after making your changes.
If models with omnilights light up the whole map after conversion, make sure you're using version 1.04 of MdlxConv.