DC offset for AC signal
The usual way to create a DC offset for a function generator would be an op amp acting as a differential amplifier, in the normal 4 resistor 1 op amp style, the function generator output would be the inputs to the differential amp with 1 small change



the ground for the resistor on the + input, you would instead connect to the output of a second op amp to buffer what ever voltage you want to offset it by,