Yeah, unfortunately, to take full advantage of this technique your content pipeline needs to be setup to use this from the start. However, there are somethings you can try. Have you looked into outputting gradient maps from the textures you already have? You can do this in Photoshop by going to Image > Adjustments and then select the Gradient Mapping option. You can then use the color picker to pick the most important areas of color to add to your gradient map. Keep in mind that doing this will likely reduce the colors you can use due to how many samples a single gradient map can use. Here are a few more blogs that I talk more about what you want to do.