Yes, I did this but it has been some time ago. I remember it being a little confusing but if memory serves you have to go to the PHONE icon and then "drag" a contact to the bottom of the screen.
I am not in my car today, but will try it tomorrow and see if I can recall the sequence
It is a little confusing - but I will try to describe what I did.
-First go to your phone contacts
-Select the contact you want to add to favorite button
-Use finger to swipe up from bottom (I think you need the bottom menu showing to do this. So if you have the arrow on right, you need to activate that first)
-Once you swipe up from bottom, you will get the list of favorite buttons
-Hold a button until it turns red
-You can then got to Settings>Manage Favorites if you want to edit
Hope that helps