Yes it is possible.
Look up sl_customization(cm) on MATLAB's documentation.
You need to write a function whose name must be sl_customization(cm) and put it on MATLAB's path. then issue savepath in command window so that it will remain on the path if you restart MATLAB.
In the sl_customization(cm) function you can add new items to Simulink's menue items and tell Simulink what it should do when each of the custom menu items are clicked.
Generally it is in the customizing the Simulink environment in the Simulink documentation help.