The View Toolbox Pane provides a means whereby existing views can be added as sub-views into a view being created in the View Composer. You simply find the view that you want to insert and then drag it with the left mouse button from the toolbox pane into the Arena or into the appropriate level in the View Hierarchy.
Note that the View Toolbox is simply an embedded version of the View Browser.
If you use a normal drag operation from the toolbox, you will create a copy of the dropped view. Often it is useful to use an Alt-drag operation, which will drop a reference to the view being dragged into the view being composed. Reference views are reloaded from their original templates each time they are opened.