If the license plate textures are inside the livery files, you want to do the following:
1. Extract the texture file containing the license plate texture inside it (ex. cvpi_sign_1) to an easily accessible location.
2. Open the file you've just extracted using any photo editing of your choice. (ex. paint.net, GIMP, Photoshop, etc.)
3. Replace the license plate image with whatever texture you want. Here's a set of SA Exempt Plates: https://www.lcpdfr.com/files/file/17085-lore-friendly-sa-exempt-vehicle-plates/
4. Save the file.
5. Drag the file you just edited into the .ytd that it came from and save.