In the Objects tab of Skin Renderer (Top left), there is a Studio Setups group. This allows you to use an Infinity curve, or a Wall and/or Floor in your render. As with any object, select the Eye and Render icons to make each object that you want to use visible.
To change the Material properties of one of these objects, right click it, and press the Pin icon in the top right section, where the options are to change the Weapon Skin. This will change the contents of this section so you can change the material for the active object. To change this back to the Skin settings, right click the Weapon/Knive/Glove, and press the Pin button again.