Yes, that is exactly my problem: unfortunately my case is complicated because I am adding new faces to the model,
I think you will have to create a new UV map then. I am not sure if there is a way to add new faces to an existing UV layout in wings without creating a new UV map. If the new faces aren't on the UV map they won't get textured at all.
obj and how those Python converters do not handle objects with more than one texture,
I believe this is more of a limitation of the OBJ file, I have read that they only support one material so the python converter will only apply that to the model in the conversion process.
I understand that, but what I actually called frustrating is how cumbersome is the process of converting dat <-> obj and how those Python converters do not handle objects with more than one texture, not the fact that adding faces result require a new UV-map. The issue is that, if I understand correctly, there is a limitation with those Python conversion programs that make you have to either import dat models that use one single texture or import a dat model ignoring textures. And then there is the cumbersome workflow for editing an existing model: dat -> obj, import obj and save as wing, export it back as obj, convert back to dat, perhaps edit it manually as to add the textures back to the file.
Your frustration is understandable but its not really the converters, I have the same issues with OBJ files exported from one model utility and imported into another, eg makehuman ->blender (no conversion software involved).
Please don't give up, the texture packs you created look awesome.