Yes, not easy. Since the grey tank is under floor, the lines drain into it from above. Those lines could be rerouted under floor to dump into the tank in its side. Since grey water does not have a solids issue, grading is not a high concern. The u traps under shower likely protrude under the floor so its moot to offset the lines under as well.
You would have to remove the tank, have a side inlet plastic welded into its side, the top fill inlet welded over. Contact a good auto body shop for plastic welding. No glue will work on Poly Ethylene. A rubber flexible coupling to connect pipe to tank will work and allow for vibration and temp changes.
