If you create multiple projects and then later decide that the content spread among the projects belongs in a single project, you can select the projects in the Library inspector or in Projects view and merge them into one project.
Merge projects in the Library inspector
In the Library inspector, select the projects you want to merge by Shift-clicking adjacent projects or Command-clicking nonadjacent projects.
Choose File > Merge Projects.
A dialog appears, indicating which project the selected projects will be merged into.
The content in the selected projects is merged into the topmost selected project in the Library inspector.
Merge projects in Projects view
In Projects view, do one of the following:
Select a project or projects, drag them into the project that you want to contain the other project or projects, and click Merge in the dialog that appears.
Select the projects you want to merge, choose File > Merge Projects, and click Merge in the dialog that appears.