When you add the source root to the project, you cannot use it again in another project. If the source root is already used in another project, the IDE will not permit you add it to the new project. If you want to use the source root in multiple projects, set it up as a separate project and add it to other projects as a compilation dependency between projects. See Setting Extension Library Dependencies.
The IDE creates the project folder containing existing code, source packages, and libraries. You can view, create and edit the project source code in the Source Editor.