while it's not impossible, it will be a bit difficult to get the spine to plane straight. In game or even in real life spine is not design to completely straight out when we are lying in a solid flat ground. you will have to play around a bit with some sliders and manipulators in spine & body morphs section in the pose editor. like mentioned above hips also help to get this pose right. There is no single slider or a manipulator to get this straight. also use butt squees sliders in the Butt & TR section to squeeze em a bit like in real life. That big booty of her definitely doesn't help in this case, although I hate to see them get flatten out.