You can create your own custom beat maps. As for the base game I have no idea, though I'm sure it'd be possible
yeah, you can, you need a decompiler for the goddot engine and the goddot engine, if you mean the tracks, then yes, there is a tool to create them, you get some reference from the discord and can also be helped by the community, other than that, installing the mods is easy as click and drag to a "mods" folder