I have seen and done similar additions a couple of ways.
The first was a thin strip of putty rolled out on a piece of talc covered glass first and trimmed to the required size. Then you lift and add to the figure and crease/fold etc. with a toothpick or other blunt thin object.
The other was a piece of lead foil placed on figure and painted in place. Cheapest source is wine or champagne bottle foils after consumption. A secondary is the safety seal of Tylenol or other medication packets.
The putty used for the first was the often referred to DURO it's from the U.K. however....
LePage makes the same thing for North America. I have found this in Canada at Canadian Tire and also at the Home Depot in Canada and in the U.S. Two-part ribbon epoxy is what it's called.
It is usually in the plumbing section or fasteners. It comes in two parts that when you mix together equal parts gives you one uniform color and it adheres great to the styrene and plastic of figures. It can be sanded, shaped, sawn into etc. when cured and takes paint without any prep work.
I have used it to replicate shell dressings on figures and bandages on heads or to add a "stuffed" look to the figures pockets etc.
Hope this helps.
Cheers;
Gregory