Line level with respect to GND
You don't need a charge pump for this. Charge pumps are used to move energy, here you're only interested in a signal (voltage). BTW, a charge pump would leave nothing left of your signal. Adding a voltage can be as simple as a resistive adder, or, more sophisticated, you can use an opamp summing amplifier.  



The advantage of the opamp circuit is that, because of the virtual ground, input signals (your audio signal and the offset voltage) don't influence each other.

Note: if you want to use the digital potmeter to create a volume control, that's not a good idea. Volume control potmeters are logarithmic, this one is linear. For a volume control the CS3310 has excellent specs, like click-free switching, though it's not cheap.