One potential problem with using Xcode for this project is the open source project may not support Xcode project files. You would have to create a new project in Xcode and add the files from the open source project to the Xcode project. You may have problems pushing your changes to the open source project because of the Xcode project file. If you use a text editor like Visual Studio Code, Sublime Text, BBEdit, or TextMate, you wouldn't have to deal with Xcode project files.
If you decide to use Xcode, start by creating an External Build System project in Xcode. The External Build System project is in the Other section of the New Project Assistant. Creating an External Build System project will give you an Xcode project that uses make as the build tool if you follow the defaults.
After creating the project add the files from the open source project to your project. You can either drag the folders in the project to the Xcode project or choose File > Add Files to ProjectName in Xcode.