When we come to the elementary constituents of matter, we come to the quantum mechanics regime and the special relativity space time description. In classical physics, masses are conserved and additive. This is not true in the microcosm of atoms, molecules and particles. There masses are the "length" of the special relativity four vector , $(E,p_x,p_y,p_z)$ , and are not an additive quantity and are not conserved. It is energy and momentum that are the conserved quantities. By contrast, charge is an additive conserved number characterizing elementary particles.
In elementary particle studies one has discovered elementary constituents of the proton, for example, which is composed out of three quarks and innumerable internal particle exchanges, which conserve charge and other quantum numbers. The mass of the proton is the "length" of the sum of the fourvectors of the innumerable constituents.
But what about mass? Is there any kind of such mass that every other mass can be seen as superposition of those basic masses?
This is where experimental and theoretical research are at the moment: it is a four vector addition that will define the mass of a complex system, not superposition, because mass is not a conserved quantity.