This feature is available in Catalyst, Enterprise, and Ultimate editions.
Even if a project has been closed, you can reopen it following the instructions in this article.
Select Admin > Projects & Fields Admin > Projects, and then click on the + expand the Project Tree.
From the Filter menu, select Show Closed Projects. The closed project name displays as
Select the closed project, click on the arrow next to the Add Child Project button, and then select Reopen Project.