I don't know of any existing ways to change the 3D dice in Roll 20. I think the closest way to do custom 'dice' would be to prerender out transparent videos of a dice tumbling along a surface. There are a few problems with this though. For 1, you'd need a different video for each dice, and would have to also have a different video for each side of the dice too. Then, you'd have to determine the number randomly somehow and drag and drop a video onto the playfield. Unfortunately, doing something like a rollable token and using an API to change the side is likely not going to work because the API breaks animated files.