How do I add System Apps to menu items toolbar?
A simple way to accomplish what you ask is to use AppleScripts. The result will be a Status menu represented by the script icon as shown below. (Hint: for a better view, click the image.)




Using the Finder, navigate to your home folder. From the Finder menus select View → Show View Options. Make sure "Show Library Folder" is checked off as shown below. (Home folder was shown in list view)


Create the ~/Library/Scripts folder, if it does not already exist.
Open Script Editor application. In the Preferences for the Script Editor, check the box "Show Script menu in menu bar" and uncheck the box "Show Computer scripts". An example is shown below. The script icon should now appear on the menu bar.


Using the Script Editor application, create a single line script for each menu item you want to appear in the scripts menu. Each file name should be the same spelling as will appear in the menu. Save each script file to the folder ~/Library/Scripts.

For Activity Monitor:

do shell script "open -a '/Applications/Utilities/Activity Monitor.app'"


For Automator:

do shell script "open -a /Applications/Automator.app"


For Calendar:

do shell script "open -a /Applications/Calendar.app"


For Network:

do shell script "open /System/Library/PreferencePanes/Network.prefPane/"


For Screen Rotate, you are on your own. I assume this would be similar to System Preferences.

For System Preferences:

do shell script "open -a '/Applications/System Preferences.app'"


For Terminal:

do shell script "open -a /Applications/Utilities/Terminal.app"

After saving the script files, the menu items should appear in the script menu.