I'm afraid that you can't customize that icons because the they are created using a different system of the classic one used in HKEY_CLASSES_ROOT\.extension\shell\open\command
in which the information of which program is executed and extra data (like menu name and display icon) where stored directly in the registry, where you can modify it with the Registry Editor.
Those icons are created using a more advanced method in which each program register a .dll than handles all the stuff related to all the menu entries of that program, including the icon. New programs tend to use this system because is more flexible and provide more funcionality that the old one (for example this how WinRAR can have an entry for Add to "filename.rar"
using the name of the selected file, with the old system it only could have the generic Add to archive...
).
So the only option for customizing those icons would be if the offer an option like WinRAR do.