Currently, adding a customized mouse shortcut is not possible in Simulink. For navigating in a Simulink model, you can use ‘Esc’ button to go to parent of active subsystem or referenced model. Also, ‘Alt+Left arrow key’ and ‘Alt+Right arrow key’ can be used to navigate within the Simulink model.
For more details, please refer this documentation link on keyboard and mouse actions for Simulink Modeling.
Hope this helps!