How do I import/add an existing Python file to a PyCharm project?

I'm not sure if I get what you want, but there's way you can add existing source into project: File -> Settings -> Project structure -> Add Content root -> choose folder with existing code

Copy the files to some directory under the project root using your favorite file manager or add the directory containing your files to the project using Settings (Preferences on Mac) | Project Structure | Add Content Root.