While in Customize, with the shortcut toolbar of interest displayed, select the icon that you wish to have the 'separator' appear in front of, press MB3 and toggle on the 'Begin a Group' item, as shown below:
John R. Baker, P.E.
Product 'Evangelist'
Product Engineering Software
Siemens PLM Software Inc.
Industry Sector
Cypress, CA Siemens PLM: UG/NX Museum:
To an Engineer, the glass is twice as big as it needs to be.