There is only one way to get an icon (well kind of an icon) for Siri, and that is to enable assistive touch.
First go to settings/general
Then go to Accessibility.
Then go to Assistive Touch.
Slide the slider at the top of the screen to enable Assistive Touch.
This will add a small button on your screen that when touched will pop up a menu that has an option to activate Siri.
While not an optimal solution it is a work around if you must have the option to activate Siri without depressing the home button. I hope this helps!