Yamaha DGX 640 (how to load new piano voices, sounds)

Hello. My name is Obed. I would like to share a problem I have with my digital piano. It’s a Yamaha DGX 640. I’m not really happy with the sounds of the Grand Pianos. The Grand Piano of the DGX 620 model sounds really satisfying to me than that for DGX 640. That’s why I looked a lot to download new piano voices that I could use, even through a USB port. But It s didn’t work. The memory of the DGX 640 does not provide the possibility of carrying out such an operation. However, I also know that nothing is impossible in Information Technologies. There are always possibilities somewhere to force things to make the impossible happen. This is the reason why I ask for your assistance in order to see, even friends who are gifted in this matter, so that they can help me. Thanks for understanding.