Voltage level shifter using opamp
You are looking for injection of DC bias into the AC signal. The simplest circuit includes capacitor, and two resistors. Look at this thread for related discussion.

Operational amplifier may be needed:


at the input into the summing circuit to ensure summing circuit will not impact the previous stage output and to lower the impedance of the output (of the input to the summing circuit);
at the output of the summing circuit to ensure high impedance and least effect of the downstream circuits on the summing quality/accuracy.


You may or may not use decoupling capacitor; if you use it ensure you have proper frequency response of the resulting HPF (high pass filter). If you do not use it ensure that constant  currents created within the summing circuit are within specifications for connected circuits.

You can use operational amplifiers in unity gain mode (G=1), if you do, ensure their stability in this mode (usually can be found in datasheet). Summing amplifier will typically attenuate signal a little; and in general you may choose not only add a DC bias to the input signal, but also "scale" the signal to the range of the ADC by adjusting the gain of the amplifier connected to the output of the summing circuit.