Sure, but that is not something that I recommend doing. That question is the same as asking "can I modify the source of the Spring Framework according to my project?" Sure you can, if you fork the Spring Framework and then customize it but that is not something that you should be doing for your project specifically. You should instead pull in Spring as a dependency of your project just like you should pull in Broadleaf as a dependency of your project.
In Broadleaf we have a ton of extension points to customize how Broadleaf works for your own custom application. If there is an extension point in Broadleaf that is missing, then maybe we need to change the framework in some way to support the use case you are trying to implement (which we would love to hear about).