Hey there Yuta_ozawa,
We would love to support modding, however it is time consuming to build a system to allow this to happen. We would need to create common systems, asset loaders and integrate custom resources into those systems and loaders as to allow content to be extended inside the game.
For now this is not the priority on our road map, but when we get to the point that we have the time to do this, we’ll for sure create a robust modding system.