You can swap the back for standard springs and shocks but you risk the back end sticking up high in the air.
Their are no sportpack springs listed for the rear of the SW. The front has sportpack springs which are 20mm lower which is how much higher your rear will stick up in the air.
You can't reuse the springs off the nivomat, they are not firm enough and your rear will sink very low.
As you can see the only solutions to maintaining ride height front/rear are replacing all your shocks and springs for standard or 30mm drop sport springs such as eibach, or cough up £1000 for nivomat replacements.