I'd use several additional op amps, resistors. Naturally the Op amps will need supply power. Being lazy, I'll refer to other posts' circuit diagrams that contain what I'm talking about for now.
One op amp would be in a buffer configuration
Buffer op amp amplifier saturation
(no resistors, just copies a voltage) to protect the oscilloscope. You may also want to protect the original signal in a similar way depending on how sensitive it is and whether stressing it is part of your test. That covers the upstream sources. Instead of using the protected source, you use the output of the buffer op amp. The buffer essentially copies it but powers it from the op amp's supply.
Next comes the op amps for a downstream super-imposed signal. This can be achieved by an amp in the summing configuration
Is there a limit to the number of inputs that can be summed with an op-amp summing circuit?
There's some annoyance if you don't want to negate the sum so you might prefer to do a subtraction operation instead.
Subtracting two voltages using an op-amp
With that, you'll have a downstream superimposed signal and you won't draw power from the upstream sources that you chose to buffer and protect. You will however want to run numbers for your power supply - the worst cases for current running through all the resistors you used and the downstream signal. If it's too much, you will likely need to increase resistance values on that addition/subtraction amp, keeping in mind that error on a resistor is usually a % of its value.
