Can an offset:ed 9VAC be used as a normal 9VAC source?
if one wants to use the lifted curve in a 9VAC-application (i.e. feed
  it), how can one "lower" it again? That is, putting the center back at
  0V


If you want to convert a 0 to 18 volt sinewave to a sinewave that has an average value of zero volts then feed it through a high pass filter formed by a series capacitor and resistor to 0 volts.

If you want to draw power from it you need to choose the value of capacitor to be somewhat higher than if it were just a signal but, in all cases, make the RC time constant suitable to pass the signal without significant attenuation.



You can also achieve the same with an inductor and resistor: -



However, it will be more economical to use the RC network and there will be less chance of inductor core saturation with dc running through the inductor.