New items, new submenus, and new separators can be created by the buttoms at the bottom left. A little practice illustrates the basic behavior of the editor. Submenus can be created to any level.

Any particular item can be assigned a keyboard equivalent using the buttons on the right side of the menu. The result will immediately appear in the outline view. However, these assignments cannot duplicate key combinations already used by TeXShop menus. If they do, the new keyboard shortcut will be ignored.

Macros Help
Defining New Macros