Add fixed voltage to variable voltage
If you want to add two voltages together without introducing an inversion it's very easy. Here's one where three voltages are added but remember the output is an average of the three (or two or however many sources you wish to add): -



Next, you need to provide a gain stage from an op-amp so that the "3" part in the denominator is converted to "1".

In the OP's question, a gain of two would be required and this is simply achieved in a non-inverting op-amp configuration with a 1k feedback resistor and a 1k resistor to 0V: -