How to add the little plus sign that expands a cell?

I believe it happens automatically when a task in the primary column is a parent task. If you try indenting a line underneath another line in a project plan for example it will show you a minus sign in the parent task and if you click on that it will hide the child tasks and you'll see the plus sign like you do in the picture shown above.

You would just have to add rows underneath that are indented. The first picture below shows the project plan with all tasks expanded so you can see the Hierarchy of the items and the second shows it collapsed with the plus signs.