Hi there, I was looking for the same thing.
I never actually found any way to add an icon to the main cpanel, but what I did was to create a "Custom HTML" module in the Administrator area and put it in the "Status" position.
This actually places it on the top bar next to the preview button and messages. This is nice because then it's always there.
You can play around with the look and feel, but I created a basic table put in a couple of icons and linked them to CB Users and CB Subs main page.
There is also a module called Art Admin Shortcuts that helps put links in the status area as well as creating shortcuts.(
extensions.joomla.org/extensions/administration/admin-navigation/8377)
Hope that helps.