You are not required to use materials listed in B31.3 for B31.3 piping systems. Materials in accordance with published specifications such as DIN may be used subject to some requirements found in the start of Chapter III of ASME B31.3 (e.g. coming up with the allowable stress). However, you will run into issues in welding, because you don't get the benefit of qualifying similar P number materials with one procedure qualification test if the material is not listed in Section II(you have the same problem with S number materials listed in B31.3).