Have you ever watched a Lets Play on YouTube? Can you, as the viewer, change anything about the game you're watching being played?
That's what this game is. All of the images in tis VN are effectively the recordings of what someone else (the dev) did with Koikatsu Studio. You experiencing the recording have zero control over the production, and cannot interact with or change any of the base Koikatsu assets.
Could the dev change characters or alter existing ones? Sure. But how they would go about introducing and integrating the new renders would be up to them. Again, it's not anything you would have any control over.