Not sure if this will help, but this is what I do when I'm making an all-new .tex:
Find a hi_ and regular set of textures from another car (cp_bar, hi_bar_1/2/3 and bar_1/2/3 for example) in the same resolution size (1024, 512, 256) that you're wanting to make. Rename them to the new name (ex. hi_penske_1.tex), use GP4 Master to load the renamed .tex, import the new .tga's into those .tex(s), save and...viola! New textures with the correct name and .tga embedded. Quick and easy.
Not sure if you're wanting a convertor for something else...if so, then just disregard my ramblings...