I don't know what do you mean with "raw .DDS files".
A .DDS file is the farthest think from a "raw" file, assuming with "raw" you mean the term generally used in digital photography, where raw is the highest quality file you can get from the camera, which you can then edit and convert to a compressed format to share it, like .JPG. In this regard, with .DDS being a lossy compression format, it's more similar to a .JPG in digital photography, that is the opposite of "raw".
In case of the GSX vehicle customization, what you can do with the included tools, is to choose between a set of different "base" colors, and add a custom logo you can create. This is very easy, as long as you create the logo following the specs, which usually means a .PNG file (eventually with an Alpha channel ) with an aspect ratio of 4:1, so for example 512x128 is a good choice for a logo.
If you wanted to do a more complex customization, it would be quite tricky, since textures for most of the latest GSX vehicles ( all the PBR versions, for example ) are not made using Photoshop with layers, which used to be a common method for repaints. We use Substance Painter, which can take the 3D model and allow to "paint" in 3D over it, using different kind of tools and filters, which all work over multiple "channels" ( Albedo, Roughness, Metallic, Height, Normal, Opacity, etc. ) at the same time, and then generate the output textures in the format required by P3D, which will finally be converted to .DDS.
So, ideally, to create a complete repaint that won't be limited to choose the base color and apply a logo over it, you would have to buy Substance Painter, and we would have to share the working project in the Substance Painter format, which is not something as common as a .PSD file that might be eventually used with other products too, but it's specific to Substance and it's even changes at each release, so it will likely require a Substance *subscription*, since each time the program is updated ( several times a year ), the format changes and you can't open files made with the latest version with anything other than the latest version.
That's why we chose to stick with the simpler Base color + Custom logo approach, which is way easier to use for everybody, and doesn't require an expensive subscription to a commercial product.