Adding DC offset control to a function generator
I believe the simplest way to try is to add a battery in series.

However if it will not suit for some reason, open the datasheet for LM358 chip (or other popular operational amplifier) and browse down to "DC Summing Amplifier" circuit. It will simply have this chip and few resistors around. You can sum or subtract signals and multiply them by any coefficients you want with the help of proper resistors.

The only limitation to this solution is when you want high frequencies (far above 1 MHz) or high voltages (chip will work in range up to 30 Volts).