This would require rewriting the action of the checkout button, click here to view unminified source code.
It’s possible if you have the stomach for it, but I would strongly recommend against investing too much time on it. Version 0.3.1 is coming very soon which will wipe any changes you make, plus I am currently refactoring the javascript to make it more modular and extensible.
In the future this kind of thing will be possible using hooks just as you would in WordPress or WooCommerce, but for the moment my focus is getting features into the base plugin.
If you would like to request hooks and discuss development please create an issue over at GitHub.