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Adding an existing file to a project is quite easy; you right-click Ex9_08 in the Solution Explorer tab, select Add > New Item from the pop-up menu;then select a header file to add it to the project.

This adds a new item. Use Add > Existing Item... to add an existing file.

Note: It appears that the existing file is not copied to the new project, but linked. This means that if you alter the added file in one project, the alteration will appear if you go to the second project.