Simulate the sum of signals on PROTEUS
Put as many different signal voltage sources as you need in series. Each one can produce an output voltage that is totally independent (or not) of the others. Putting them in series ensures that the overall output terminals is a voltage that is the sum of all the individual voltage sources. Connect to the scope input.
Or, if you can't put them in series, connect each signal to a voltage controlled voltage source and wire the output terminals of each VCVS in series to create the same effect.