[4.13] Importing Source code into a Blueprint project

Hi there,
I’ve been working on a blueprint project and have realised that I’ll need access to some of the default classes in the engine, so I’m keen to add the source code to my project. Tricky thing is, my current blueprint project is already connected to BitHub. Is there a way of me merging the source code files into my project? Or will I just have to bite the bullet and set up BitHub for a new project with the Source Code files and migrate my blueprint stuff over?

Sorry, I’m not very techy so I lack a bit of confidence with this stuff.