What do I add to my sugarpaste to make it stretchy?

I have bought a coloured sugarpaste to cover a bag cake. I am making a "duffel/tote bag" cake and want to "gather" the icing at the top of the bag slightly to give the appearance of a duffel bag. I want it to stand upright. Will the icing stand on its own once its dry or will I need to knead something else into it before covering the cake?