Yes and no.

You can write custom widgets for the game (check out the discord channel #widgets for more on that!) but at this time we don't support full moddability and customization.

We will be supporting it but at this time the game is still in Alpha and we simply don't know exactly how it will evolve. Worse yet we don't have the developer time to support any modders. One day you'll be able to mod BAR and play your mods with friends. Until then please jump into the discord and help out with the game development. It'll both give you a better understanding of how BAR works and make the day we can support modding draw closer!