The documentation is here: https://doc.qt.io/qtcreator/creator-project-creating.html#adding-subprojects-to-projects

It says, "To create more subprojects, right-click the project name in the Projects pane, and select New Subproject in the context menu. To add an existing project as a subproject, select Add Existing Projects."

That option does not appear to exist. Screen shot attached.