The process of adding materials onto a surface in art is called:

The process of adding materials onto a surface in art is called collage. In collage, an artist glues or attaches various materials - such as paper, fabric, photographs, found objects, or other flat items - onto a backing surface to create a composition. The word comes from the French coller, meaning "to glue."

While "additive" describes any process that involves adding material (as opposed to subtractive processes like carving), it is a broad category label rather than a specific art technique. "Subtractive" is the opposite process, involving the removal of material from a solid form. Collage is the specific, established art term for the technique of adding and adhering materials onto a surface to create a unified artwork.