You can add any number of signals in any ratio using an op-amp. If all the input resistors are equal, then the signals are just added but if the resistors are not equal, the signals are scaled and added. But beware of the phase shifts after the amplifier or the capacitor (as seen above). Phase shift is frequency dependent.
Resistive addition is the only practical way to go about this, but it may need some buffering either at the input or output, depending on the impedances involved.