Carbon Fields can be easily extended. Each of the Field classes is optionally declared, and each function is optionally defined. This way early custom declaration and definitions can be made in your theme or plugin. Also, you can create custom Fields, Containers, Exceptions and others by extending the existing PHP classes.
To get a better understanding of how the library works, checkout our step-by-step guide of how to create a new Carbon Field type, which can be found at https://github.com/htmlburger/carbon-field-template. The guide itself also acts as a template that you can start from.
In addition, you can also review the Number Field which is a simplistic extension field.