Adding large numbers in context of analog computing?
The high voltage needed problem can be solved by choosing the correct scaling. If you used 1 uV to represent a value of one you can work at scales of 10^6. You would however have problems generating and measuring such fine graduations of voltage.

This assumes that your circuit is ideal. In practice the resistors and opamp have tolerances that will severely impact the accuracy of your results if you are trying to work to this precision.

If you need accurate arithmetic at scales of 10^5 go with a digital representation and adders.