To Create a New Sub-Menu

Do one of the following:

Select File > Settings > User > Customize Ribbon .

or
Right-click in the blank space in the ribbon and select Customize Ribbon from the pop-up menu.

The Customize Ribbon dialog opens.
In the list below Customize the Ribbon, navigate to the split button or drop-down button in which you want to create the custom sub-menu.
Click New Sub-menu.

A new sub-menu is added in the selected split button or drop-down button .
(Optional) In the Properties section, set the following:
1. To change the label of the menu-item, enter the new label in the Label field.
2. In the Key-in field, enter the key-in that should be executed when the menu-item is selected.
3. To change the icon, in the Icon field, click the Browse button on the right-side. The Define Icon for Tool dialog opens, from where you can select the icon for the new button or menu-item.
4. To set the visibility, select the visibility expression from the Visibility drop-down list.
  Note: The Visibility property holds the name of a named expression that will be evaluated each time the active model is changed to determine whether the item should be displayed. The named expression should return a boolean (True or False) result in order to work properly as a Visibility property.
5. Set other fields, as desired.
Click Apply.